The primary use I see are;
1. Have a clone backup of your primary drive. (Carbon Copy Cloner, SuperDuper, Time Machine).
2. Have an extra storage handy for a rather economical price without lugging around an extra drive.
I was also planning to have a bootable Linux on one of those, but I never got around to actually doing that. Now that my 2012 MBA's flash storage is dead (and here in India either I can't find replacement, or repair shops are asking for half a kidney) I will see if I can actually use a card instead.
Personally, all my devices are endpoints or tools. For instance, if I lost my phone, it will take me a few hours or a day/two at best to get back to where I was. The same goes for my other devices (desktops, laptops).
In the most critical situation, say, I lost my phone while traveling, but lucky my wife's phone is in-tact, I can pick up hers and go through my contacts (we sync our contacts two ways). I can then login to anything anywhere by getting to her Password Manager, which fortunately has a copy of mine. I might need to ask her to authenticate it for me.
I strongly feel that one should have a pattern and system of redundancies and be independent of the devices one uses. I'm still discovering and learning a lot; not yet the best, but I'm happy I started quite a while back.
I will still have data on rsync.net, b2, Tarsnap, borgbase, some even in Dropbox. Also, the weekly backup to my external hard disk.
That card is more like regular/scheduled backup sink (encrypted data) like those cloud destinations I mentioned above to help with laptop malfunctioning or just dumping some data on it quickly when I want to take it with me (that's rare though).
I too am a bit confused by the non-popularity of such a setup. SD cards are okay for rarely changing data (or data that changes little, exactly like the incremental backups) and are extremely compact.
I view SD cards as the diskettes I always wanted to have some 26 or so years ago.
This stuff is just like firewire. Maybe it will be different in 5 or 10 years.