In a nutshell, trying to get components to "work" at their full potential. These days, it's really easy to "tune" the components to your liking - the darn things almost do it themselves
Why doesn't the companies just "release" them at their best settings?.. Has a lot to do with the unpredictability of "other" components in your system. It's not really feasible to run a CPU at a higher clock speed with the heatsink/fan they give you in the package.. Also, you have no idea what the underlying mobo is going to be.. The specifications are pretty standard.. But usually the companies make the components "more overclockable" for those people who have the right tools to do it with.
One would argue that this is a business decision as well.. Sortta trying to "please" the overclockers if you will. In every batch of components they manufacture, there will be ones that are greatly overclockable, while others not so good.. But pretty much all of them "could" be overclocked "somewhat" more "assuming" the right environment (case, fans, etc.).. But, the companies leave it to us since they have no idea what we have
Anyways, your question was not re: the business merits about why the companies do things.. So I'll stop