MReverb is a special reverb designed for mixing, particularly spatial positioning.
When you listen to a good mix, you should be able to place each sound source at some point in space easily, otherwise all instruments will appear at the same position and the mix may sound muddy. When you are recording in a good room with well placed microphones, you may not need any processing, but this is not always possible. Professionals use panning, delays and reverb to simulate the room. But you need years to learn it and after that you still need a lot of time to perform any change.
This is exactly what MReverb is for. You specify room properties and then you just need to move the sound source where you want and MReverb will try to make it sound like the object is in that spot.