i cannot speak for everyone, as there are truly too many reasons people choose one solution over another in the computing world as you probably well know. However, on the subject of overclocking, i believe i understand at least 2 of the biggest draws for that particular process, and further believe they encompass a huge percentage of the overclocking "communities" reasoning behind doing it.
1: There has always been a market for people who desire quality and performance without paying a price premium. The less expensive anything is the better, we all know that..if the "better things in life " were all affordable that would be awesome. That, however is not the case usually. So consumers want to get as close to that "premium consumer good " as possible while maintaining their budget. The basic principal behind overclocking comes down to a mindset of " i cant afford a $500 chip, so i will make this $100 chip work as best i can. " Over time, overclocking has grown out of an option from necessity to practically a given, and stability has grown with that development over the years. It has reached a point where the tight budgeted users are not the only ones looking to squeeze extra performance out of lower priced parts, the financially sound have seen the capabilities, and have wondered why they should spend $500 on a chip that their roomates $100 overclocked chip is outperforming. Gone are the days of putting racks of 486 DX2 66's into a Maytag fridge with nitrogen pumps to see if they will oc before they melt through the boards, a little time, research and minimal additional parts yield amazing results these days quite safely.
2: Enter the enthusiast. When explaining computer hardware/software concepts to people who are not exactly technically savvy, i often use automobiles as analogies. It does the trick so well it's frightening. Owning a computer is very much like owning a car. The people who need a little 4cyl to get back and forth to work are like people with emachines. I wouldn't try and sell my granny on the idea of needing to think of DDR3 and the new Nehalem chips when all she does is check her email and look at her church website. Granny, buy that emachine, and get that warranty that will allow for a swap out so you dont ever call customer support, you know? Just like car owners over the years, there are back and forth to work cars, family workhorse cars/vans/SUV's, specialty vehicles like semi's and dump trucks, and good old American muscle cars and import tuner cars, there are computer builds that run similar usages. Dredging up memories of guys and girls back in the 50's and 60's spending Saturday mornings in their garage polishing valve covers and adjusting throttle bodies and timings on their favorite Ford or Chrysler to hit the road Saturday night, they wouldnt trust ANY mom and dad's mechanic near their baby! They knew that car inside and out, could break it down to a frame and put it back together in a day, and tune it to purr. Why though? Doesnt it work just fine without doing all that? If that's your line of thinking you missed the whole point to begin with.