"sound guy" is always the most underrated role, yet they can be more specialized than doctors. there's a group that specializes in recording cars. they'll place mics all over a car to get the perfect recordings. ask those guys to mic up a person for a quick interview, and they'll become so offended at your request and refuse. some people speicialize in music instruments and within intstruments you get people that specialize in drums, horns, etc. dialog people are all over in their skills too (booth, on-set, wireless, boom, etc).
Now, getting into SFX, folley, and other stuff gets just as crazy too. Some people record actual sounds, some bang out noises on synths (meant in a polite term of endearment), etc.
of course, just like in any industry, some just show up long enough to collect a pay check and and fiddle with knobs while baked.
For me, it was one of those things that I was clearly taking for granted until I had to do it myself. My first thought? "We need a sound guy"