+ It's a always +EV to play a freeroll, because to put it simple, you can only win.
+ They are good to learn the rules and some basic concepts of the game, hand strength, positional awareness, bubble etc.
+ For new players
freerolls are a lot of fun.
- once you reach a certain level of lets call it "intermediate" skill, freerolls won't help you get better anymore. Further knowledge has to be learned the "hard way".
- If you run bad, freerolls are usually more frustrating than games where money is involved, simply because of the crazyness of your opponents. The fact that they crush you while playing terrible occurs quite frequently.