While new players can be bought after you first game, rerolls cost double! So you might as well buy them when they're cheap - you'll need them eventually anyway.

Rerolls can be used by almost anyone in your team no matter what they're trying to do - a goblin blocking a treeman, or a Mummy attempting a really long pass to score. In that sense they're more better than skills like block or pass. The exception is Big Guys, who can't use team rerolls at all.