Most of my photography involves contact printing on silver chloride papers and my preferred film developer is ABC pyro, which to my knowledge is not available premixed. Ditto the amidol formula I use for printing and the fixer (2 bath plain hypo, with sodium metabisulfite added to the first bath). Although they're all available as kits, I find it far less expensive to just keep a supply of the raw chemicals and mix them ad hoc.
I don't mix my own because it gives me any particular advantage. It's just that they're not available premixed or are prohibitively expensive premixed. When I develop negatives I intend to enlarge I usually use Defender 777, which is only available as a kit of pre-measured powders (when it's available, which it doesn't seem to be at present).