Given you don't understand that the core pillar of the product manager role is to be the owner of the problem space, I'm not sure how qualified you are to comment on how valuable our role is or isn't.
Discovery isn't about decided what does or doesn't get built, it's about discovering what the real problem is that your customers need solved (almost like it's in the name).
If your PM is good at their job, the answer to that question should be pretty clear once they're done. That's not them "telling you what to do", if you want to go build a solution to a problem nobody actually has, you have fun with that.
And if you PM is defining solutions and telling your team how/what to build, that's on you to push back and take ownership of the part of the process that you're meant to be owning.
A lot of PM's end up overreaching because they're just tired of there being a leadership vacuum and nobody willing to fill it. Trust me, we're busy enough, we don't want the extra work.