Modern CPUs have become more efficient over time, and have begun to have lower cooling requirements. As a result, the CPU manufacturers have designed some rather advanced stock coolers and are either supplying them alongside their top-tier CPUs or selling them as aftermarket solutions. Despite the fact that these are the 'certified' coolers for the processors, the CPU manufacturer has to make millions, to every hundredth of a cent in manufacturing can be important to the bottom line. It is not easy for the average user to assess just how good the stock cooler really is and how much of an improvement, if any at all, there will be from the purchase of an aftermarket cooler. End users need to be aware of the performance of their current cooling solutions in order to reasonably assess the upgrade that will fit their needs..
Great article comparing all the stock CPU coolers.