AMD

so why another 3D cache boost for 8 cores again????

Binning and validation. If they have enough of a chip that doesn't meet the binning and validation requirements to run at a target/baseline clock speed and voltage, they have two options: one is to throw it in the bin after the expense was accrued to manufacture it. The other is to validate them at a lower frequency and voltage and sell them at a lower price, giving people a new option while still turning a profit. Seems pretty obvious why you'd choose option 2 instead of 1.