If you have a server lets say it has 2 CPUs, each CPU has 8 cores each, and you install SQL... be it 2008 R2, 2012 or 2014.... can you tell it only to use certain cores?
Case in point... app server that requires 2x8 core cpu, but the db it uses to store data works fine with 2x4 core cpu. But for the server to function without performance degradation, to run the app and the db, it needs 2x8.