Is there some way to make soul gems out of fragments or something? Could someone explain the point of this item?

No, you can't make new soul gems out of the fragments.They are just minor loot

Yeah, they're pretty much just for show.

The first time I found some of these things I thought there would be a way to make Soul Gems out of them, but unfortunately that turned out to not be the case. Which was rather disappointing, since they show up in multiple locations and do respawn if collected. Of course the long respawn timers mean that it's unlikely you'll ever collect a given lot more than once, which may be part of why they were left as-is.

There's a mod that lets allows you to craft soul gems out of the fragments ...

Yea, there should have been some way to craft soul gems out of the fragments (how many determines the size of the soul gem).

Yeah, and I'm using it but I'm not sure which one it is - maybe part of deadly dragons? I've lost track, I really have, 104 esp/esm modz, 10 SD/SKSE plugins & every decent retex worth having. Skyrim folder 13.9GB and rising!
Anyway, the fragments (three of four different 'flavours' even if they look exactly the same in the inventory) can be used to make small dragon soulgems, those in turn make dragon soulgems, which can be used to improve the 'dragonslayer' armours found as loot on dragons, or to make some special spellbooks & items.

...but I think the 'vanilla' fragments are in fact totally useless. Another ball dropped by Beth to make the deadline?

I've considered doing this myself, with a higher number of fragments for each tier of gem, but I'm undecided as to which crafting station to add it to. Probably the Enchanting altar, although that would require finessing its recipe format...

yes there is a use, a rather important one at that, looks.

You should be able to use Soul gem fragments to make Black soul gems