Multipart archives still exist. They're now used for file sharing websites that have a maximum file size. Before that they were for unreliable p2p networks, so you didn't lose the parts you'd already downloaded when your peer goes offline. Originally it was to fit something big on multiple cd-roms or floppies.
Opening somthing.rar also reads the data in somthing.r01 through somthing.r15 etc