Well, now stock coolers don't come included with K series processors so people buy a cooler separately. as for before K processors used to come with stock coolers but they are not good for overclocking. People buy different "Better" coolers for lower temps or less noise or even better looks. these generally don't effect performance unless you overclock of course which can't be done with stock cooler because it will cause over heating. but stock coolers are totally fine on stock clocks.