more specifically, it's trying to copy the sucess of the CS/MA nature of ethernet -- instead of trying to establish a constant connection (with guaranteed throughput), experience has shown that it's better to have a "collision domain" where everybody competes for temporary access. If two requests collide, they both back-off for a random amount of time and try again. As long as the signaling speed is fast and you don't have more than about 25 requestors, you get consistent transfer rates (60-70% of the theoretical).

P.S. I know I've got the acronym for this technique wrong but I'm tired and don't want to try and look it up tonight...