Great when you first get the machine or are formatting a drive.
Splitting the drive in two will allows you to keep either two different OSs, or a backup on the smaller partition.
Some of the updates cause problems, so having a backup handy is sometimes great. And also handy for repairing your main boot volume.
Especially if you are apt to throw away some important OS files...
Basically it just splits the drive in however many parts, each with its own file system.
In the old days it was easy to frag the file system and while the SCSI drives tended to last a long time, these days the drive is likely to die quicker.
So backups are still essential.