Balance, balance, balance, and safety. If you have your foot straight, you have a bigger chance of getting hung up on the stirrup. A well-trained rider will really only have his toes, or the ball of his/her foot in the stirrup.
If your unable to push your heel down (Western riders!) then your stirrups are to long. I noticed that on my Western saddle, on my Aussie, I have such amazing balance because the stirrups are *perfect*.
If your horse bucks, you can clench those legs and stick more like velcro instead of ending up on your teakettle