I'm only talking about Windows here. Not everyone wants or needs 2 OS's, but in appropriate circumstances, this overcomes hardware or software compatibility problems.
For example, you want the new functions and stability of XP, but you have a game or program that doesn't run (or doesn't run well) in XP, or you have a scanner that doesn't have an XP driver. You keep your old OS like 98 to continue to use the old software/hardware/peripheral.
Or you keep the old OS as a temporary measure until you're sure XP works.
The other incidental advantage of having 2 OS's, IF you use the inbuilt dual boot functionality of NT, win2k or xp (i.e. you only have one active partition), is the relative convenience of backing up either OS.
When you're in one OS, the system files (except perhaps the page file - which doesn't need to be backed up anyway) and more important, the registry, of the other OS are not in use. The other "OS" becomes just a collection of files and folders. Since the other OS partition is not hidden from view (because there's only one active partition), you can back up the other OS/partition quickly and easily without having to go into DOS. Restoration is the same.
I use win2k, and have my main working copy on C. I have a second copy (a "bare" win2k, no programs - but you can configure it the way you like) on a separate partition. I boot into my 2nd copy to back up the entire C partition. This way, all the system files and registry files of my main win2k are copied automatically. I also make a floppy which can boot me into the 2nd copy of win2k in case the boot files on C get corrupted.
I just use MS Backup for this purpose. It doesn't copy useless files like temporary folders and the big page file, or the hiberation file, if any, but has its other limitations (like, restoration depends on you being able to boot into Windows - which is a big disadvantage). But there's no reason why you can't combine your favourite backup software with this method to avoid going into DOS, if that's possible with the software.
As I said, it's an incidental advantage if you re going to have a dual boot anyway. Whether it's worth installing a 2nd copy of an OS (taking up all that space and time) just to back up the main one is quite a different question. I've done it but I'm sure it's not for everyone.