Another reason why Rocket artillery was really popular with Soviet commanders was that it was, in combat, pretty easy to manage (to an extent even logistically, tube artillery ammunition is cheaper, but tube artillery itself is a lot more high maintenance), and responded well to centralized planning. Rocket artillery was often used as the personal "shotgun" of the commander on corps or higher level, and, because it was pretty mobile (was on a truck, and you cant really put tube artillery on trucks because them tubes are heavy), could potentially murder anything it hit (including tanks that were breaking through), and if you had "enough" (which the Soviets pretty soon did), then just hitting a grid square was completely sufficient as far as accuracy went, because with enough rocket artillery you just "remove" the grid.
Bonus points because their speed in unloading their payload, combined with the above average mobility, allowed them to shoot and scoot to an extent.
The Soviets (and also the Germans) did use Rocket artillery in close quarters btw. Aim 500 or so meters away from your own guys in the direction of the enemy, and chances are very good that you will mess up them much much more then your own soldiers. Even if you dont hit the direct frontline, well, that barrage is going to mess up communications, reserves, somewhat behind deployed anti tank, infantry mortars, you name it.