Two reasons. First, because during the initial transition from cable, you might want to keep watching existing shows from where you left off, without having to wait for the end of the season. And second, because other people watch the show live and you want to keep up with them.

The first problem goes away after the initial transition, and the second goes away when everyone you know also uses Netflix and/or Amazon Prime. But don't underestimate the value in getting people to switch.