There are a multitude of reasons, you will find out more and more as you play.
Getting within range for melee or ranged attacks, out from combat, into range with scheme markers, giving out free moves etc.
This game is a lot about managing resources, where your models AP (action points) are the main resource. A push could be a free move, letting that model have, in effect, an extra AP his next turn.
It's also a way to move models outside of the normal restrictions, like severe terrain or out of combat (without risking a disengaging strike).