It's all about information. Key information about a player that has a sore arm, going through a breakup, death in family, distracted by owing money or not having any when bills come due is what it's all about.
And of course, there are always those players that are paid serious money to affect the outcome of a game, a missed block here or a botched kick there, maybe a dropped pass at the end of a meaningless game (yet, point spread may be changed as a result). Believe it because its true, not so much in the Pros, at least as far as throwing a game, but much more prevalent than you might think at the college level.
Bettors get an edge when they gain access to that information, it's really a dirty business when bookmakers pay serious money for that information and then sell it to bettors looking for it.
Sure glad I don't bet!!