https://en.wikipedia.org/wiki/Standard_(warez)
Edit: another major use case overlooked here: a considerable amount of media applications will stream files from within a RAR archive without manually unarchiving beforehand, making them more accessible from file storage sites like mega, 1fichier, without carrying the external appearance/negative baggage of an mp4 or mkv.
Of course there were still people who screwed that up. I remember downloading some software once that came in the form of a multi-volume RAR, maybe 20 floppy disk-sized RARs. Download all 20 of them, get them in the same directory, then un-rar them, and lo and behold out the other end comes a single .RAR file that was inside! So I un-rar that single RAR, and out the other end comes 20 separate floppy disk images. Obviously the "scene" distributors weren't always the computing world's best and brightest...
Just...why?
The BitTorrent protocol will handle corrupt data and fix it at the segment level. You won't have to redownload an entire file.
Splitting a file into pieces made sense 20+ years ago when connections (Both your physical ISP connection and the logical TCP connection) were unstable, web servers didn't always support download resuming, and software didn't handle graceful unexpected disconnections, but those days are long behind us. We transfer data over encrypted channels that include checksums at the packet level. We use software that can handle disconnections, automatically reconnect, and resume where it left off, not to mention detect when data went bad and re-download just the bad part.
There's just no damn reason to split a 5 GB .mkv into 100 50 MB .rar files which will then take my poor RPi 15 minutes to decompress.
Heh, "ancient"... Geroffmylawn, you punks.