Here is what I'm referring too. If I were to buy my processor now, it would cost me: $239 for the AMD64 3200 939 90nm. Now, my processor is presently overclocked to 2.4ghz as opposed to it's stock 2.0ghz. At 2.4ghz I'd be spending $630 on the AMD64 3800 939 chip. So by overclocking I would have saved myself $400 (roughly). Not too bad. Now since my Powersupply isn't able to handle everything properly, I have a drive and 3 case fans that are disconnected to make sure my system has enough power to be stable. If I had a better powersupply which is part of my upcoming upgrade, I could go to 2.6ghz or higher. That would make my processor the same speed as the AMD64 4000. Which is currently priced at $719. That's pretty close to the $500 marker.
Now instead of handing over $719 for a processor which will become outdated within the next few months. I was able to save myself nearly $500 by purchasing a lower level processor and overclocking. Now that's just the money reason.
As far as practical use. With a higher speed, I can launch programs faster, extract and encode files faster. Sure, the difference is only a matter of seconds. A minute tops. But it's satisfying to know I have that extra speed in there.
For lifespan, Instead of my processor becoming a lower level processor as newer programs come out, my processor will give me that much extra time where it can run and utilize those programs for me to move onto my next upgrade.
I don't condone the purchase of the latest and greatest processors and technology. Any true overclocker rarely does. You wouldn't be overclocking if you did. Not saying that the latest and greatest can't be overclocked. But a lot of overclockers look at overclocking as a way of achieving the latest and greatest without breaking your bank account. Whereas other overclockers like to break their bank account to achieve what is so commonly refered to as... the E-Penis factor. Breaking benchmarks with latest and greatest overclocks. I find that to be assinine.