The adapter allows you to use an AA (LR6) battery instead of a C (LR14) battery.
It is 12mm tall: you can stack 4 of them and get the exact battery height, but if you want to save plastic you can also use only 2, or even a single one if you are in a hurry.
Compared to things 39478 and 4205:
less likely to scratch the battery labels (eneloops scratch very easily with the old design).
height adjusted such that 4 of them stack to an exact battery height.
sturdier design but more flexible inside to allow for big variations in AA battery diameter: the inner triangle works like a spring.
simpler geometry: a bit faster to print, 20% taller but still uses the same amount of plastic
a bit more resilient to shock and breakage
Print in ABS, 0.33mm layer height.
Print lots of these and save $ and waste.
I enjoy designing minimalist-functional 3D objects.