One advantage is that you can have fresh stock in just the quantity you need at any time. Another major advantage is that you can mix up things that are not on the market commercially or are hard to find. The disadvantage is that there are many excellent proprietary formulas that are not available. A minor (or maybe major) advantage is that you can experiment.

I don't do that much black and white work any longer, so I value being able to make D-76 as I need it. I also make Defender 54-D for prints, which I prefer to Dektol (D-72, which you can also mix). I also mix up F-34 as a cheap, near neutral, non-hardening fixer. Stop bath is trivial and very cheap. From time to time I experiment with other things - a lot of other things, including alternative processing. There are a huge number of formulas available out there, and you can find something appropriate to any of your black and white needs.