Here are a few uses that I came up with in 60 seconds:
1.) Gain reach.
2.) Gain improved grab/constrict/etc ... and other special attacks.
3.) Disguise oneself.
4.) Increase movement for short durations.
5.) Flight.
6.) Underwater action.
7.) Gain traits of the form such as low-light vision (animals) or immunity to poison (plants) [Note - this issue is still not 100% clear, but the designer, Andy Collins, is on the record as saying that you gain extraordinary type traits of the form via polymorph]
8.) Healing (my sorcerer certainly enjoys this aspect).
9.) Increasing strength without using an enhancement bonus.