Overclocking has several motives behind it
In order of I feel the necessity,
You can OC to eliminate bottlenecks due to a slower processor, such as I did with my 965 and 760.
Next is to squeeze that little extra bit of performance out of the component for some easy labor (Maybe 1 FPS, make take 20 seconds off a render time, maybe more of each).
Then there are the people that do it just for the speeds and namesake.