Those explanations are certainly good and related to the question, however it all stems from scene rules. All the groups who actually distributed those things, called scene releases. All follow the same rules for packaging things, if you do a google you will see what I mean. The multiple part files actually come from originally the BBS days of when you’d upload them to a scene board. It’s alot easier to upload many files vs huge file on a modem.
Take a look at this Wikipedia entry which gives a good outline about it.