I mean why have two different ways to launch, tube and rocket ?
Torpedoes are slow and have limited range.
An ordinary tube-launched torpedo is just ejected over the side of the ship using compressed air or similar. It is essentially a reactive, close range weapon.
With Yu-8 and similar weapons, the torpedo is attached to a booster rocket that travels through the air up to a couple dozen kilometres from the ship, where the torpedo detaches and enters the water. It is a more proactive weapon suited to engaging submarines at longer distances, potentially queued by other ASW assets such as the shipboard helicopter.
If you tube-launch an ASW torpedo at a distant submarine the time it takes to get there will give the target plenty of time to launch its attacks, and plenty of warning to maximise its chances of escape, while using up most of the torpedo's fuel stores before it even arrives at the area. Yu-8 arrives in the target area much faster and drops the torpedo (ideally) right on top of the target, giving it minimal opportunity to escape.