Step 1: To start off - Why do I need a air compressor?
1. pump up a tire
2. change a tire(with a impact wrench)
3. inflate you favorite water toy
4. paint a car (with a paint sprayer)
5. put on a roof(with a nail gun)
6. power a spud gun
7. Make: something
8. Because its cool
I could make this list go on for a while. Anything you use your shop air compressor for you can use this on-board air compressor for.
You will probably find a use for it you never thought you could use a air compressor for! I know I did. (supercharged a charcoal grill)