and every team that wants to trade back gets to.
Every team has needs, but many teams are desperate at many positions (ie, there doesn't have to be a once in a generation QB, or even a stud OT or DT for trade value). The Cowboys traded up to get a CB and the Falcons gave up FIVE(!!!!!) picks for a WR! Different teams fall in love with different players at different positions for all kinds of different reasons. With a savvy GM, the possibilities are mahy, including simply staying put and getting to chose who WE want.