I have no idea if that's why it was done that way but it sure seemed to fit well. I'm pretty sure usenet is still a popular way to "pirate" TV shows and movies and such. It's just not as well known since it typically requires you to pay for access these days with most ISPs dropping support (and I doubt they'd allow access to the TV/movie groups anyway).
After CD-RW drives became common RAR files were mostly used for uploading and downloading as you describe, to avoid file size caps and to make it somewhat easier to fix corrupted chunks. By the time the data being shared was filling the CDs we had writable DVDs, so RAR wasn't needed to span CDs. Then writable BluRay came around before DVD spanning became necessary.