Most often I've seen instances use as super communities. Largely revolving around a bigger topic. KDE runs their own with their own subcommunities for instance. They are far from the only ones. Just the one I use the most and a came to mind first. Having your own instance slap server allows you far more control over your communities then just hosting on someone else's server. But from an end user perspective very largely transparent. not even being noticeable oftentimes.