Utility of Delta G

What is the main utility of delta G? As far as I could understand, if we do not know whether the reaction we are dealing with occurs in nature or not, we would not be able to tell in which direction it will proceed. Delta G will only tell us whether the reaction with compounds on one side being A + B and compound on other side be C + D, whether A+B will react to give C+D or whether C+D react to give A+B. Am I right?   
  

(If possible please explain in simple language. Not a chemistry student)




$\Delta G$ is a convenient way of determining the spontaneity of a reaction that takes place in constant temperature and pressure. As you might know, a positive $\Delta G$ would mean that the reaction would not be spontaneous under normal conditions, and the opposite would go for a negative $\Delta G$.


Your assumption about what we determine the outcome of the reactions to be is only half-correct; there are always direct or indirect methods to compute enthalpies and entropies. 


Thermodynamics isn't everything though. There is **kinetics**.



> 
> Thermodynamics should, as Professor Zare noted, really be called "thermostatics." Thermodynamics is not about things moving and changing but instead about how stable they are in one state versus another, while kinetics is about how quickly or slowly species react. It is dangerously easy to confuse thermodynamic quantities like free energy with kinetic ones like activation energy. Probably for this reason, thermodynamics and kinetics are often taught separately. – [Stanford University Site](http://web.stanford.edu/~kaleeg/chem32/kinT/)
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Imagine a mixture of hydrogen and oxygen gas. Thermodynamics favors that hydrogen and oxygen react:
$$\ce{2H2 +O2->2H2O}$$


However, this reaction won't happen unless you provide **[activation energy](https://en.wikipedia.org/wiki/Activation_energy)** in the form of some heat, spark etc. IOW, this is a famous reaction for having a high **kinetic barrier** despite thermodynamic favorability.


Take into account the reverse reaction: $$\ce{2H2O->2H2 +O2}$$


This reaction is non-spontaneous. Does that mean it won't happen? It means it won't happen **by the pathway deduced**. But you can use electrolysis to make it happen. So being $\mbox{non-spontaneous}$ only means the reaction won't occur as you now have written it.


To sum up, the only thing $\Delta G$ can tell you is that whether by your proposed mechanism the reaction will proceed or not. How fast that might be is pure kinetics. Note that the usual famous $\Delta G = \Delta H - T\Delta S$ only holds for constant temperature and pressure.


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