I sort of know. Based on my limited understanding, multisite networks allow a bunch of sites to share some common elements for ease of management. For instance, the sites could use the same theme and plugins setup. I imagine, there would be ways to have them share a single media library, but I have not looked into that yet.
Right now, I don’t have a real use case where multisite is going to be useful, as far as I can tell. At my current level of experience, what I mostly want to do is explore various themes and plugins and there’s very little call for changes in one of my sites to be automatically shared with the other sites — on the contrary, it’s best if the advertised sites like ‘campaign’ remain exactly in their current state while I make learning experiments elsewhere.