I've found another use for split - putting very large files, in my case dmg archives from a Mac, onto a FAT32 formated USB external HDD. Xcode weighs in at 4.1GB. Too big for FAT32.
In addition to the email example above, I could imagine if rsync'ing or scp'ing large files over a WAN, you may want to chop the file up so you can restart with out starting from scratch.
The split program of course exists on Mac's (basically BSD) as well as Linux.