Old carpets, curtains, even paper egg containers... All that finds its way into rehearsal rooms in a usually vain attempt to improve the room's acoustic and keep the loudness at a reasonable level by damping reflections.
Mind, this particular foam is not professional studio-grade thick, altghough it would certainly help in a small bedroom studio.
But it's cheap and effective to improve any band's rehearsal room which is why I bought several pieces. And it does the job - sound improved immediatelly, it's clearer, more refined and more detailed. Suddenly, people are not trying to overcompensate for not hearing themselves by turning the volume knob to 11.
A word of advice - if you don't have a long-term rehearsing place, you may want to avoid glueing the foam directly to a wall. Instead, use wooden frames or wires to hang them on the wall. This way you'll be able to take them with you anywhere.
In any case, experiement with the placement - every room is different. Also, you don't want to cover the entire surface of all walls and ceiling - some reflections are good for you.