I mix my own chemicals because:
I can tweak the formula to fit my needs. E.g. if I mix Amidol paper developer I can add a little bit extra of Kbr (Potassium Bromide) to make the image warmer looking. Or when mixing ABC Pyro film developer I can add some extra Sodium Carbonate to speed up development. This is useful when I need push more contrast.
Especially for the formulas I use, there are big savings mixing it oneself. Another point is that I can mix exactly the amount I need, not needing to have bottles of unused chemistry on the shelf for too long!
I like the feeling of being able to mix everything myself, gives me some self-confidence in what I am doing, even If I'm far away from being an expert.
I actually mix everything from raw now: film developer, paper developer, stop bath and fixer. I will also in the future mix hypo clearing agent as well!