You use the generated parser as a platform for experimentation.
If you know the language, and you have a bunch of users, and you are writing a parser for it, by all means, write a parser by hand and give it the best error recovery that you can muster. If you are developing a language and want to do a bunch of experiments, it pays dividends to use a parser generator. And then there is the whole space of DSLs and mini-languages you encounter, where beautiful error messages are a nice-to-have, but you would rather ship a generated parser and move on to more important work.
It’s easy to focus on compilers from the perspective of familiar languages like writing compilers for Rust or for OCaml, but you may end up writing a compiler that gets used by a much smaller number of people, for smaller tasks.