The main reason for partitioning is to have filesystems with different characteristics. For example, defragmenters will let you choose different schedules for partitions, so you can defragment the Windows system partition (which Windows scribbles all over continually) more often than a partition that holds only installed applications and large downloads.
If you dual boot with Linux, Linux is traditionally set up with multiple partitions, and partitioning is more useful. In this case, it's also handy to have a secondary NTFS partition that both Windows and Linux can write to; this avoids potentially troublesome configurations like mounting your Windows system partition read-write from Linux.
If you dual boot with Linux, Linux is traditionally set up with multiple partitions, and partitioning is more useful. In this case, it's also handy to have a secondary NTFS partition that both Windows and Linux can write to; this avoids potentially troublesome configurations like mounting your Windows system partition read-write from Linux.