“Harmonic Function” is music theory coded language. I believe it was invented to describe Western Art music of the classical period. It’s a little post-hoc.
The essence of functional harmony is to find harmonically pleasing ways to get to the V chord (or other dominant) so it can resolve. That is just one aspect of harmony among a lot of different kinds. If you’re asking “do working musicians use this?”, the answer would be different among different types of working musicians. Jazz musicians, I’d imagine are frequently thinking in terms of a kind of functional harmony. ii-V-I is a meme for a reason.
I wouldn’t say you need to learn functional harmony to bypass any musician gatekeepers. It’s more like a way of describing a certain kind of pattern that is actually pretty unanimous. But classifying “functional harmony” as a discrete thing is probably less useful than theorists would like to think it is.