Very few applications can load 8 cores uniformly. Moreover, the operating system may "park" some of your cores when they're not needed. This is important because on AMD CPUs and Intel CPUs with hyperthreading, pairs of "logical cores" share some hardware resource with each other. So if you have mostly two threads of execution running of say AMD CPU with 4 cores, you don't want them to run on cores that share L2 cache with each other.