A) it is for bragging rights and pride (it's a 100 year-old rivalry for heaven's sake, b) we are each other's only true rivals, and c) a loss in that game could potentially keep either team out of a national championship game (example: Utah wins the PAC-12, but because they lost to BYU in September they lose their shot at a national championship to a BYU team that won the Big-12 and beat all their other non-conference opponents).