There are different ways to set up a drive depending size, the one or more OSs being run, the amount of files as well as types of programs being used, and of course the user preferences. Those all have to be weighed in.
Intially Vista was on one single primary until later deciding to shrink that to see a second backup partition created in the event Windows was reinstalled. The user/admin account's data is by default seen on the OS primary while the paging file and system restore points can be assigned to a second partition or even a separate hard drive installed.
That tends to keep more drive space available for the main OS as well as less volume improving performance to some degree. Scheduled system backup would go on either the second partition or drive in case Windows sees an "ut ooooo" for some reason. Your personal data and settings are safe guarded there.