The Rigger is one of the most important assets in any team. Their jobs can include, but not limited to:
1) Transportation. Vehicle riggers are often the teams way in and out of a run, maybe under fire... The importance of this goes without saying.
2) Overwatch. This includes monitoring the team and the target, usually via drones. Where as the Hacker keeps an eye on the Matrix et al, the Rigger often watches the target and keeps tabs on what is going on when real-time eyes-on intell is needed. Better to lose a drone than a team member if a trap is sprung.
3) Support. As already stated, this includes infiltration when eyes-on intell is needed before or when a team goes in. It can also include direct fire support when things go south.
4) Technical. Need something fixed? Chances are any good rigger is also a decent mechanic/engineer and maybe can pass as a secondary gunsmith, armorer, and techie. And if they dont know this stuff (which is in my opinion a poor rigger), by nature of their jobs, they usually know someone who does. Gotta get their toys somewhere, eh?
Now, I've played several riggers. I tend to go for the general specialist rather than being a dedicated vehicle or drone rigger. I say, why not both? I usually try and have several vehicles for a variety of jobs. This includes one "main vehicle" that is part command vehicle, part fire support vehicle, drone carrier, and part getaway. Also good to have on hand are dedicated assault, getaway, and throwaway vehicles if you don't want to lose "your baby".