Because it unlocks even more performance,Im not comparing my Q6600 to an i7 as theres a huge diffrence,however just as an example my Q6600 was 2.4Ghz stock....i overclocked it to 3.5Ghz and its much much faster.
Btw,Overclocking your cpu will not realy improve slowdowns when you host a game im MW2...thats to do with your connection and everyone elses who is in the game.
I have 20mb unlmimited broadband,and when i host it gets laggy too....thankfully i dont have to host that much.
Gives you that little boost think that you've gone and modified you computer and made it faster! And also i7s power is only really released the higher you go.
Data can only be transferred as fast as your clock speed throttled by your bandwidth. CPUs work in cycles, and if one component is way faster than the other it creates a bottle neck.
When I was younger I over clocked all the time, now I don't see the point as it doesn't really improve actual real world performance, only benchmarking performance and maybe in some cases high end computing.