Some photographers like to be in control of every aspect of their art, and it gives them the full control they crave. I suppose my reason is that I like to tinker with things. Mixing my own chemicals give me more choice in which developer to use. I can use obscure or specialist brews. I can make enough of a formula to test one film, and if I'm not happy with it, I haven't wasted money on a whole bottle of chemical.
Some photographic chemicals are toxic.
Always take care when handling chemicals and wear suitable protective gear, such as gloves and safety glasses. If you are unsure about what safety precautions to take, consult someone with chemical handling experience. All the formulas here are presented for information only, and come without warranty. Use them at your own risk. Ok, providing you are sensible and preferably have some experience in handling chemicals, there should be no real problems.