Grouping players into tiers helps drafters make decisions about when to pick players from different positions, especially after the very early rounds, when the "best player available" approach often makes sense. For instance, if you need both an RB3 and a WR3, and you see that the group of RBs you still think are start-worthy is drying up, while there are plenty of good-looking WRs left, then you know to go with the ball-carrier over the pass-catcher.