Top mount position is a control position.
In this dominant position, you have a significant advantage over your opponent, both in control and in striking.
The opponent is flat on his back you are sitting on his belly.  You can break down the position by using three different sitting positions.
From here, you can choose to transition into various techniques that maximize your control and effectiveness.
The first is the high mount your body is forward on your opponent, your knees are under his arm pits.
This strategic positioning also opens up opportunities to set up submissions and further enhance your control.
This position eliminates your opponent that ability to pop his hips and sending your body forward in hopes to lock down an arm and roll you off of the mount.
It can also create openings for significant damage without much risk of retaliation.
The high mount gives you the advantage as a great striking tool.
Additionally, maintaining a solid base while striking ensures you remain difficult to unseat from your dominant position.
Generating power with good posture allows for powerful striking with punches and elbows.
These techniques require a keen understanding of leverage and body mechanics to be executed effectively.
Simple manipulation of the opponents arms can be performed by using your knees.
Effective use of these techniques can often lead to a swift conclusion of the match, whether through points or a submission.
Applying pressure with a straight arm to the elbow pins the opponent and stops his ability to defend against punches or elbow strikes.  

.
Overall, mastering the top mount position is crucial for anyone looking to excel in grappling or mixed martial arts.