It's good for offloading things that otherwise eat useful fast storage.
For example, OneNote uses a cache and a backup folder. So whatever size your notebook is, it will consume 3x that storage space.
I use the SD slot for the cache and backup folders (my backup folder is synced to a file server, so I don't need it locally, and in 15 years of using OneNote, I've needed that backup one time).
It's also useful for temporary stuff that you don't care about/is available elsewhere. I'll pull large installers from my file server and put them on the SD, until l I get around to using them (laptop drive is 250, which is tight for me, and the SD was a quick, dirty solution since I have a bunch of micro SD's from phones over the years).
If you set it up properly (like using apps to sync folders) a big enough sd is like local "cloud" service.
I was thinking about it recently, after my phone data were very close to being deleted (I managed to prevent it eventually), I was angry at how not having an sd slot caused me so many issues. If I had a 1tb sd I would just autosync app backups and files to my card and not worry ~at all about losing data from bootloops etc.
maybe a rpi-style headless micro computer with 2 or 4 micro SD slots: tah-da: supertiny, supersilent NAS with 0 peripherals except power cable, yet has redundancy and is just as small and portable as an external drive.
I would happily use one for my music and movies to access them on the go. I already have copies elsewhere, so it would be no big loss if the card died.