Filament spool adapter.

prusaprinters

Filament spools come in a variety of formats, the biggest variable being the bore in the middle. When the bore is much larger than support it is mounted on it may swing around. Combine that with the movements of the printer and non linear feeding, and you can get quite big movements. To reduce this, a smaller difference between the bore and mount is used forcing the rotation of the spool.There was an effort to reduce the amount of material used and also does away with supports for printing.(The one in the picture was printed using a creality years back, hence the quality)