One of the main reason one would clone a drive is upgrading your drive in the computer.
let say you have a 500GB boot drive that is nearly full, you can clone the drive to a larger drive like a 1TB one. the nice think about cloning is that when done right once you replace the old drive with the new one, one should be able to boot of the cloned drive without having to reinstall anything.
most hard drive manufacturers provide a free version of a cloning software. WD offers Acronis.
This is very popular: http://www.macrium.com/reflectfree.aspx
Closely followed by this: http://www.paragon-software.com/home/br-free/index.html