The banner is basically a first impression of your comic! Rather than the first impression being whatever page happens to be the most recent, you can make sure the cover banner introduces your comic the way you want it to be.
It's also one of the very few ways you have to customise your page, which makes it look more like Your Comic's Page rather than just a collection of comic strips.
I don't use music on my comic, and I almost never click on it when other people have it. I personally don't feel like music integrates well with comics in that way, so I don't bother with it!