What is the benefit of praying istikhaarah when things are already decreed?
Question

 
With regard to praying istikhaarah: it crosses one’s mind sometimes: 
What is the benefit of asking, supplicating and striving, when the decree of Allah is what will come to pass?.
Praise be to Allah.Allah has made du`a a means of attaining what one wants. The Lord enjoins it, as He says: 
“And your Lord said: ‘Invoke Me [i.e. believe in My Oneness (Islamic Monotheism) and ask Me for anything] I will respond to your (invocation). Verily, those who scorn My worship [i.e. do not invoke Me, and do not believe in My Oneness, (Islamic Monotheism)] they will surely enter Hell in humiliation!’” [Ghaafir 40:60]
Once this is understood, there will be no confusion, because Allah has decreed things according to their causes. So if it is decreed that a person should have a child – for example – it must inevitably be preceded by marriage and intercourse in order for the child to come after that. The results cannot happen without the causes. The entire universe is based on the connection between cause and effect.                                       
The same applies to du`a or Istikharah too. 
Allah has decreed many things that are connected to calling upon Him or asking Him, and the desired outcome does not happen without the cause or means, which is du`a, in addition to tangible causes. The ahaadeeth of the Prophet (peace and blessings of Allah be upon him) clearly point to this idea.  
It was narrated from Ibn ‘Umar (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “du`a may be of benefit with regard to what has already happened or what has not yet happened, so adhere to du`a, O slaves of Allah.” Narrated by al-Tirmidhi (3548) and classed as hasan by al-Albaani in Saheeh al-Jaami’ (3409). 
Shaykh al-Islam Ibn Taymiyah said in Majmoo’ al-Fatawa (8/69): 
If a person says that he does not say du`a or ask of Allah, because he relies on the divine decree, he is also erring, because Allah has made du`a and asking means of attaining His forgiveness, mercy, guidance, support and provision. If good is decreed for a person he will attain by means of du`a what he cannot attain without du`a. What Allah has decreed and knows with regard to His slaves’ circumstances and destinies is only decreed on the basis of means, and decrees will be fulfilled at the appointed times. There is everything that happens in this world or in the Hereafter happens on the basis of cause and effect; Allah is the Creator of both cause and effect. 
Disregarding the principle of cause and effect is contrary to reason. End quote. 
He also said: (8/287): 
One of them said: du`a is nothing more than pure worship, because what is decreed will happen, whether one says du`a or not. It was said to him: Allah has made du`a a means of attaining the desired outcome that has been decreed, so how can it happen without du`a?! End quote. 
Ibn al-Qayyim (may Allah have mercy on him) said in al-Jawaab al-Kaafi (p. 4): 
du`a is one of the most beneficial of remedies; it is the enemy of calamity; it wards it off, remedies it, prevents it befalling, alleviates it or reduces it if it befalls. It is the weapon of the believer. In the case of calamity, one of three scenarios must apply: 
1.It (du`a) is stronger than the calamity, so it wards it off;
2.It is weaker than the calamity, so the calamity overpowers it and befalls the person, but it may reduce it even if it is weak;
3.They resist one another and each impedes the other. End quote.
Shaykh Ibn ‘Uthaymeen said in al-Majmoo’ al-Thameen min Fatawa Fadeelat al-Shaykh Muhammad ibn Saalih al-‘Uthaymeen (1/157): 
du`a is one of the means by which what is asked for is attained. In fact it wards off the divine decree and nothing wards off the divine decree except du`a, and that may happen in two ways. For example, a sick person may pray to Allah for healing and be healed, and were it not for the du`a he would have remained sick, but he was healed by means of the du`a. But we say: Allah decreed that the sick person would be healed from this sickness by means of du`a, and this is what was decreed. He thinks that were it not for the du`a he would have remained sick, but in fact he did not ward off the divine decree, because the basic principle is that du`a was decreed and that the healing would come by means of this du`a, and this is the original decree that was decreed from eternity. The same applies to everything that is decreed to happen through some means or cause. Allah has made this means the cause for the thing to happen, and that was decreed from eternity, before it happened. End quote. 
The scholars of the Standing Committee for Issuing Fatwas were asked: Does du`a repel the divine decree? 
They replied: Allah has prescribed and enjoined du`a. He says (interpretation of the meaning): 
“And your Lord said: ‘Invoke Me [i.e. believe in My Oneness (Islamic Monotheism) and ask Me for anything] I will respond to your (invocation)’”
[Ghaafir 40:60]
“And when My slaves ask you (O Muhammad صلى الله عليه وسلم) concerning Me, then (answer them), I am indeed near (to them by My Knowledge). I respond to the invocations of the supplicant when he calls on Me (without any mediator or intercessor)”
[al-Baqarah 2:186]. 
So if a person avails himself of the prescribed means and calls upon Allah in du`a, then that is part of the divine decree, so it repels one decree with another, if Allah wills that. It is proven in a saheeh hadeeth that the Messenger of Allah (peace and blessings of Allah be upon him) said: “A person may be deprived of provision because of sins that he commits, and the decree is not warded of by anything but du`a, and nothing increases one’s lifespan except righteousness.” End quote. Fatawa al-Lajnah al-Daa’imah (1/195). 
They were also asked (24/243): 
Does du`a reduce calamities? Does Allah show kindness to us as the result of du`a? How can that be when Allah sends down calamities upon people despite the fact that they call upon Him in du`a? 
They replied: 
du`a is an act of worship towards Allah, and Allah has commanded us to call upon Him, as He says (interpretation of the meaning): 
“And your Lord said: ‘Invoke Me [i.e. believe in My Oneness (Islamic Monotheism) and ask Me for anything] I will respond to your (invocation). Verily, those who scorn My worship [i.e. do not invoke Me, and do not believe in My Oneness, (Islamic Monotheism)] they will surely enter Hell in humiliation!’”
[Ghaafir 40:60]
“And when My slaves ask you (O Muhammad صلى الله عليه وسلم) concerning Me, then (answer them), I am indeed near (to them by My Knowledge). I respond to the invocations of the supplicant when he calls on Me (without any mediator or intercessor). So let them obey Me and believe in Me, so that they may be led aright”
[al-Baqarah 2:186]. 
du`a may reduce calamities, ward them off or ward off greater calamities. The Prophet (peace and blessings of Allah be upon him) said: “Nothing wards off the divine decree but du`a.” If calamities happen, they expiate sins and raise people in status, so if a calamity befalls the Muslim, he should bear it with patience and seek reward with Allah, and he should not complain about the divine decree. End quote.  
From these quotations we may derive an understanding of the issue in sha Allah. When the Muslim understands the issue of Istikharah and du`a as a means of attaining what he wants, he will never neglect it and he will never try to attain what he wants in any other way. du`a will become a source of strength and a way to good for the Muslim, as Allah wants it to be. 
For more information please see the answer to question no. 11749. 
And Allah knows best.