The seat slides forward to give the 3rd row more legroom(which it needs desperately) or to create more cargo space. The rearmost position is where it would be in a "fixed" location in an X without 3rd row.

As a side note - I believe you also get the self leveling suspension with the 3rd row(at least that's how it worked on my '09). Not sure if that is still the rule. Havent kept up with options since I'm not looking to buy another X anytime soon. Now I'm on to comparing 3er options.