Same here. You don't even know what you're worth if you keep working at the same company. And even then, if you don't know what you could ask, you will never know how much you're really worth.

Let's say you could command 100k/year but your first job is 30k/year and stay there for several years because you like the work environment. Then you'd go to the next company and ask for, I dunno, maybe 50k/year and think that's a huge bump. If you saw salary ranges for similar positions elsewhere you'd be more inclined to start looking around even if you were happy at your current employer.

Nowadays things are a bit more transparent in the tech world with stuff like levels.fyi (but you have to know about those), but I'm thinking it would benefit people in all fields.