Where I used to work we pretty much fueled everything on the airport... our ramp, two different flight schools, airlines, highway patrol, military and so on. We spent a good portion of the day on the move driving around the airport. Pretty much like what everybody said, fueling, towing, LAVs. Customer service is huge, things like making sure aircraft have catering, ice, coffee, and newspapers on time may be the difference if they come back again or pick a different FBO the next time around.