The main reason I still have one is that it’s still the best way to store a really large digitised movie library, and music to a lesser extent. It’s faster, cheaper and more permanent than leasing the equivalent space, and will also still work if your internet goes down. It’s also more private, which is good should you have lots of stuff obtained nefariously over the years that you don’t really want to stick on Dropbox or whatever.
Secondly, they’re just another fun bit of tech to piss about with. The Synology ones are essentially mini PCs with all sorts of apps that you can install - multimedia stuff like Plex for video transcoding, Photo library apps, as well as a ton of very geeky stuff that I don’t really need/understand.
I still store all my important stuff (my photos, accounts related stuff, work related stuff) on cloud storage.