I want to understand who they are for and how to use them

If your work has nothing to do with design, you probably won't need it. So do not bother yourself. But if it's related, start with the video instructions.

I use it all the time, I'm a designer. But sometimes it's not enough to come up with a beautiful design, you need to present it correctly so that the client likes it and wants to make fewer changes. Mockup helps to make a convincing presentation. I take a mockup on https://mockupfree.co/indoor-advertisement or create one myself. But if you're not a designer, it's unlikely you'll use it.