Changing the way player animates

I read several forum threads and resources incuding plaer.cc source code, got to know that player animation transtion is based on its velocity.

1) Why does not it directly change when move key('w', 'a', 's', and 'd') are pressed? Just for a simple physics?

2) Is there any other resources which does not use player's velocity to change its animation?(eg. walk to run etc.)