It's actually useful in cases you might not expect. Take a look: https://themes.evolvapor.com/
When a new .ecigtheme is uploaded, I have a background task that checks for new uploads, generates preview images using our software, and inserts the new images in the database. If I were doing this with the filesystem, I'd need to give this task write permission, and the ability to reach, the directory on the server hosting IPB.
Regarding S3... Being in the vaping industry -- and really, this is true for anyone in an industry entrenched, wealthy interests would like to be rid of -- we try to be self-reliant where we can. You never know that you won't wake up the next day and be disconnected for some reason invented the day before. For us, and many others, "better solutions like S3" effectively do /not/ exist today.
If I were able to cache these, the hit rate would be 99%+.