This is a tool for programmers who need to fill out a lot of PDFs automatically. For example, a freelancing service could use FormAPI to fill out W-9 forms for contractors.

Or if I built a service for end-users to fill out and sign PDFs, I would create a separate website and use FormAPI to generate the PDFs. (I don't think I will do this, though.)

You're right that if you just need to fill in a single form, then there are easier ways to do that. (Even just using the Preview app on Mac).