They weren't very accurate, the smoke from the rockets gave away their firing positions, and their long reload time combined with the need to displace to avoid counter battery fire meant their sustained rate of fire wasn't that great.

However, they could fire their whole load of rockets in less than 10 seconds, while it would take a 105mm howitzer 12 minutes or so to put the same amount of explosives downrange. They could be mounted on a truck chassis whereas the weight and recoil of a tube artillery piece meant that SP guns mostly had to be based on expensive (and thirsty) tank chassis. The noise they produced when fired en mass was absolutely terrifying, which enhances their shock effect.

They were also basically just big model rockets, and could be produced by a factory that wasn't much more advanced than a high school metal shop. Meanwhile producing a tube artillery piece that's accurate and which won't burst while firing requires a bunch of very precise work by well trained people.

Multiple Launch Rockets were not as versatile as tube artillery, and would not have been very good substituted on a one for one basis. However, their ability to drop a huge weight of explosives onto a wide area almost immediately combined with their low cost made them very effective for certain situations.

The Soviets produced something like 10,000 of their MLR systems during the war, so they certainly found them useful.