Squeezing 2% of the file size seems like an argument. But it isn't. They rar the thing and split the rar because that's how they do it. Period.
It's the stupid habit idea. Nothing wrong with it. Files were being RARed and split long before video files had such an high rate of compression. They just do it this way because that's what they have been doing for years.
The 2% used to be important. Not anymore on non centralized p2p networks. So yeah, again, it's an habit. Sheesh!