BJJ

Hi, so for context I'm a 2 stripe white belt, and our school just did a 2 month series of classes on Knee on belly attach and defense. One thing I didn't quite glean from it is why would I do this vs mount? I if I have someone in side control and can throw a knee over them, I can usually do a full leg without much more effort, and mount is a much safer attacking position.

So I guess what I'm missing here is when do you guys go into this position?