I use folds for one specific thing: I have a general "notes" file for home and for work, which is a big outline where I put anything I'm thinking about or working on. I usually don't delete stuff, so the file grows over time. I make a fold for each top-level item (~project or subproject) and sometimes nested folds if things get too big. I always use fdm=marker. The automatic fold methods are too confusing.

Everyone uses tag files, in some sense, when they use :help :)