The importance of game 6

One of the key reasons that winning game 6 is so important for the Yankees:

If they win, CC Sabathia won't have to start game 7 in the ALCS. That will mean that Sabathia will be available potentially to start 3 World Series games; game 1, game 4 four days later, and game 7, 4 days after that.

If the Yankees lose game 6 of this ALCS, Sabathia will need to start game 7, which will be on a normal 4 days of rest. That means Sabathia will only have the opportunity to pitch in two World Series games. Those probably would be games 3 and 7 but possibly games 2 and 6, depending on how Girardi wants to do things.

Here's a link to the postseason schedule. But I can simplify it by saying:

Win game 6, and the Yankees will be fully rested with all pitching options available for the World Series. Lose game 6 and the Yankees will need to make compromises that will limit their options against the Phillies. That is, if they make it to Philadelphia at all.