I believe this is a holdover from piracy the early 2000s, when file sizes needed to remain small (due to unreliable downloads or the need to fit on a particular piece of physical media) and RAR was a convenient format for generating compressed archives in multiple parts.
I recall it being the common go-to for file-sharing (piracy) uploads on Usenet. I'm not sure if it was because of repair-ability when you were missing some of the RARs, file-size limitation, resume-ability, low bandwidth, etc... Probably a combo of them all I guess; wasn't aware it was still used nor that it might be more common in RU.
