What is the purpose of thinner airfoils used near wing tips? I know that a thicker airfoil at root will strengthen the root spar but what is the reason behind using thinner airfoils at the tip?

Is it because thinner airfoils have higher stalling angles of attack? If so, why?

Comparing both thick and thin airfoils, who generates more lift and why?

Also, my final question is that in what kind of designs would such a change of airfoils be recommended? I'm currently working on designing a high lift wing (selig s1223 airfoil) for delivering a payload of about 15kgs. with the total aircraft weighing about 20-22kgs. Would changing the airfoils in my wing design be recommended?
There are two main reasons for thinning the wing towards the tips:


Thinner airfoil sections cause less drag simply because they disturb airflow less.
Thinner sections down to 10% have a higher maximum lift coefficient, which helps both near stall and during maneuvering: A section with a higher maximum lift coefficient will show more aileron effectiveness, too.