When you overclock your CPU you place an additional load on all of your pc components this includes your motherboard, ram, graphics card and power supply. A system is not stable just because you can boot into Windows and browse Facebook. You need to verify the stability of your computer by putting it through a series of tests designed to push your system to its limits.
These stress tests or as they are otherwise known in the overclocking community, stability tests, will ensure your computer doesn’t randomly crash while under intensive processing load resulting in your potentially losing that clutch game or pivotal document you spent all night writing.