I have a bunch of marker flags - like Martin said, it’s easy to stick the wire stem into any of the various soil types in my area. But I’ve only carried them and used them a handful of times - literally only when I’ve intentionally set out to compare target signals using two different machines. I’ll mark a few with one machine, then go back with the other detector - saves a little of the switching back and forth, and of course, you don’t lose track of the target. Otherwise, I just don’t see a need to mark a target - if I’m there with one machine, I’m digging as I find stuff. No need to save it for later!
Another potential use would be to mark out a particular area or “box” on a site - it could help you stay focused within a particular “hot spot” or help you visualize an area you want to grid search.