Nearly everyone practices side-to-side footwork, but very few people practice in-and-out footwork. There are many occasions where this is necessary to contact the ball at the optimum timing. There are many many situations, but here are a few for starters....

Moving in...
Against a long-pips or anti-spin block
Against a miss-hit
When playing a chopper
After going back to retrieve a deep ball

Moving out...
After receiving a short serve
Against a punch block
When looping or smashing a high ball

Thanks to Satoko Hayashi for demonstrating this drill for us...