Developing good footwork is very important for big men as well as guards. Your big man should be able to catch the ball on the block and have the footwork to be able to finish the play without traveling. Depending on the level of player the post player may be required to do more advanced moves around the basket, but with a strong foundation of correct footwork it will help the post player prepare for whatever moves they need to learn.
Footwork and agility workouts are just as equally important for great shooters. Shooting on the move is all about the player getting their feet set, and body squared up to the basket, and this requires good footwork as well. So whatever position you play it is important to be able to have great footwork.