motion is usually used to figure out what the defense is in..........
for example, if you send your receiver in motion across the formation and the CB follows him, they are in man coverage..........
if the CB stays on his side, and someone else from the opposite side of the formation (the side the receiver is coming to) moves into position to cover him they are in zone.........
What he said....
Plus...its used to help on run plays. Draw men offsides. Sometimes a guy lines up out of place just to go into motion to the spot he should be in.