Quite a lot of examples come to mind. It's pretty much anything that would take significant time to upload (then download), if the space even exists.

I have a MacBook pro with limited storage, thus using an external USB 4 drive with 4TB. I rsync some folders from it and then back when I don't want to have it connected, e.g. during flights. It's less convenient but saves a lot of money (external NVMe 4TB was 250 + USB 4 case for 70, vs 1000 for the 4TB upgrade from apple).

I sync my 2TB Google drive folder to it + video files for editing)