Once you tackle someone, you MUST release them. The reason they are asked to ROLL away instead of simply MOVE away is because if they get in the way, then they will be penialised for slowing the ball down. The referee wants to see them, with their hands in the air, rolling away, so that he knows that the player isn't trying anything sneaky and slowing the opposition ball down illegally.
One of the fundamental principals of rugby is that when you are off your feet you are not allowed to handle the ball. When you are taken off your feet during a ruck (meaning near a grounded ball) you have a duty to move away from the ball quickly so that some player who is on their feet can play it. A common (but illegal) defensive tactic is to lie around, sealing off the ball so that it can't be quickly played by the attacking side. This especially true when the defender finds himself on the wrong side of the ruck in an offside position. Offensive players will sometimes do it too, when the defenders are counter-rucking strongly and about to poach the ball. Generally the quickest way to get way from the ball is to "roll away". Getting up to "move" away would take to long.
When they are on the ground it's quicker to roll away. It's usually when they have strayed offside. It keeps the game flowing.
