As someone that wants to learn about computational chemistry, mainly for its use in energy storage materials (and therefore DFT as the most practical method of it), I am wondering what is essential to learn about DFT. The theory behind it seems quite vague if someone does not have the practical mathematical formulations. When a chemist comes to use computational chemistry in software, it is something completely different compared to when they learn about DFT.
What I understand is that the most essential task is the improvement of exchange correlation functionals to give better and more accurate results, which must be then compared with experimental observations, but the course that I have followed never discussed any way to do it. The consideration of LDA and GGA are never clear for a computational chemist for what is better or worse to use and the development of them is a job for a quantum physicist for example.
So I am wondering, what is really the essential role of a chemist in computational chemistry and what does he/she have to learn to be effective?