In WR routes where the WR actually comes back to the QB footwork helps immensely at increasing the speed of that turn, it also helps a WR get upfield quickly after the catch like Xars said, due to Wr's almost always turning fully around to make a catch rather than catching in stride. I personally think footwork helps defenders stay in position for the tackle when a HB is trying to turn the corner on them as opposed to the defender having to turn upfield and make a pursuing tackle rather than a head on one.
I also think footwork helps elusive HB's with movement, jukes, and maintaining speed as they slip tackles. I might be in the wrong here, but I actually think a healthy investment of footwork is important for all HB's. Basically any time a player can't adjust their direction of movement enough that they need to sidestep is when footwork comes into play.
http://glb2.warriorgeneral.com/game/replay/213211/2581291?player_id=122524For instance I believe my HB shedding this tackle and not missing a step was footwork.
http://glb2.warriorgeneral.com/game/replay/213211/2581734?player_id=122524This step as well I believe was purely footwork
http://glb2.warriorgeneral.com/game/replay/211999/2394426?player_id=122524Again I believe footwork was what got him through the line so quickly
There are more replays, but you get the picture unfortunately he's switched teams along with all his OT's so I think chem has got him and the OT's playing at a sub-standard level, but we'll see as the season progresses.