MReverb is a special reverb designed for mixing, particularly spatial positioning, which has been a very hard and time-consuming task. When you listen to a good mix, you should be easily able to place each sound source to some spot in the space. Otherwise all instruments will appear at the same position and the mix may sound muddy. And this is exactly what is MReverb for. You need to specify room properties and then you only need to move the sound source where you want and MReverb will try to make it sound like the object is on that spot.