what and how and where- would it be a fun job or a frustrating one

depends if he is the tech for the whole band or just a certain player ?
either way, he is responsible for setting up - stringing, tuning, maintenance, etc of the bands/players instruments.
players can alternate instruments on stage between songs, so the tech is responsible for having it ready..... and wiping the other one down (sweat off it), & getting it ready to be reused or packed away.
they also see that they are all packaged safely for travel between shows, and that they are all accounted for (not stolen)
some techs are even experienced luthiers that can do repairs and modifications if the need should arise, like changing out to a different pickup or fixing a volume/tone pot.

I haven't been on the road in years, but when I was, I had to do just about everything dealing with the sound... from doing all the maintenance on the stringed instruments to repairing the sound equipment and I even ran the boards when the audio guys were drunk, hung over or in jail.