These are officially called scomp links. They are the standard data interface for the Star Wars universe.
The only explanation is that computers of the day (in-universe) seem to have quite a few physical components to them. For instance, we see in Rogue One that the Imperial Archives store data on some sort of drive/tape (this is a deleted scene so you can see the device better)
We also see Princess Leia get this information onto a data card and put it into R2D2 in ANH.
It's entirely possible that R2D2 is physically having to read "sectors" of data as the arm spins. Need a different set of data? Rotate the interface (just like a hard drive). As manual as these other data devices are, this would make sense.