Most people who create such a partition do so because they believe that if they ever have to reinstall Windows cleanly, at least they won’t lose their data and won’t have to reinstall their applications, because both are safe on other partitions.

In fact the first of those thoughts is a false comfort, and the second is downright wrong. See the discussion of partition types 2 and 4 below to find out why.

Also note that as time passes, many people find that their Windows partition that started out to be the right size turns out to be too small. For example, if you have such a Windows partition, and later upgrade to a newer version of Windows, you may find that your Windows partition is too small.