The CSR Room reverb was designed to reproduce the kind of reverberation generated by medium to small acoustical spaces. It is particularly useful to “dress” a dry, close-mike recorded source in a natural ambience that won't make it sound like an artificial reverb, but as it was actually recorded in a real room. This is far from easy as even though the reverberation doesn't have a long decay the presence and importance of the early reflections make it really tricky to reproduce convincingly.
In a mix situation the CSR Room reverb will make tracks fit better because of the natural “air” surrounding them or, when driven harder, place them on a different sonic plane, making them literally jump out of the mix.
Tweakable to the minimum detail it can be an invaluable tool on a tonal palette to shape mixes that will sound 3D, articulate and real, where every track fits into its own place and space.
Try it on a bone-dry acoustic guitar and listen to how the CSR Room module instantaneously makes it more real and lifelike. Percussion instruments are also a very good candidate for this processor, especially in studio recordings with a very controlled ambience that must be rendered “looser” in a mixing context.