In the mount (a), the person on top has their legs astride the chest of the person on the bottom. This is a very advantageous position for the person on the top. In fact, for the person on the bottom, this is the single worst position to be in in a fight.
The reason why this is so bad for the person on the bottom is mainly due to punching. Due to the relative position of the shoulders and faces, the person on top can punch the face of the person on the bottom (b), but the person on the bottom cannot punch the face of the person on the top.
Furthermore, the person on top has gravity on their side to aid their punch (b) and can fully cock their arm. The person on the bottom cannot fully cock their arm as it is blocked by the ground (c).
When the person on the bottom is able to use their legs to control the hips of the person on the top, then they have the top person in their guard.
In the near guard (d) \& (e) the person on the bottom has their legs wrapped around the hips of the top person. The opponent is clinched close, so that they do not have the distance to punch. The bottom person uses their legs to control the hips of the top person.
In the far guard (f) \& (g) the person on top is kept away with the knees. In this position, the top person is too far away to effectively punch at the bottom person's face.