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).