the reason comes from the old days when you could create the swap partition on a second hard disk that could improve drastically performance by unloading the constant read/write process of the swap partition from the main hard disk with the / partition that sometimes has less usage than the swap partition
that was very useful in a time when ram prices were too high/impossible to increase and a swap partition could be as small as 128 mbs in older 1 gb hard disks that you could have around doing nothing
this same trick can be applied to windows to gain the same performance increasehttp://www.techtalkz.com/windows-xp/157103-pagefile-sys-placement.html
another reason is to share this partition between multiple linux distros, this was a common practice some years ago, to save space
the swap is also a partition to avoid the situation where you don't have enough free space on hard disk and can't create the swap as a file
this days depending on your system usage you can run the system without this partition at all, this is because the ram usage in linux and the amount/price of ram this days, something that you can't do in windows
remember that swap/pagefile.sys are both extensions of ram but available on a slower device, the hard disk that you don't wish to use to avoid the noise of the hard disk moving and the slowness of the system accessing it(a hard disk is drastically slower compared to the ram of your pc) but if you need it is better to have it available isn't?
more about swap partitionshttp://www.linux.com/news/software/applications/8208-all-about-linux-swap-space