On a MTB the seating position varies slightly depending on the area of use, but overall it is relatively upright. A saddle designed for this posture provides optimum comfort and the necessary support, even on long rides. When it comes to the descent, the saddle should not be in the way, which is why MTB saddles allow a lot of freedom of movement. To find the right saddle width, it is a good idea to determine the individual seat bone distance. You can easily determine this yourself, e.g. with measuring cardboard.