There are two big reasons to prime...
1- Bare metal requires a primer that inhibits oxidation. Body shops use etching primers on bare metals, then go to sandable primers.
2- Sealing and adhesion. Top coats adhere to primer better, and certain primers have sealing properties. In body work, the previous history of the cars paint can be unknown, and there can be some incompatable paints down there that could cause issues with the new paints, so primer/sealers are used. If smoothness is an issue, then a primer/surfacer could be used over that. Then color and clear.
Thats the proper way...
In the spray can world, it really isn't that important. Test a small are if you decide to go without a primer. Look for weird "fisheyes"..areas where the paint has adhesion and flowout troubles. It looks like...fisheyes..duh! ...or crazing ...or wrinkling or just weirdness. It could be caused by wax, grease, or the dreaded silicones from earlier products that are in the paints structure. Use a good degreaser ("PreKleeno" is the R&M version...or any real paint cleaner..NAPA sells one that's fine) first, of course, and buff the shine off with a green or red scotch brite pad and you should be fine without a primer.
If you do all that, and you still get "fisheyes"...it could be embedded silicones, and that could mean that you have to go all the way to bare metal. It's rare, and I doubt you care that much, LOL.
(If anyone has seen Dave's car (which I will point out was the highest finshing Prelude in ITB at the ARRCs) you know why fisheyes in the interior won't cause him a loss of sleep!)
Oh, that's messed up! Hey, it was really pretty at one point. When I did the bodyshop sponsor deal, I brought the car home and didn't even want to race it anymore. 
