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.
If you knew what CPU binning and validation are you'd realize this is actually an awesome idea. If they have CPUs that aren't able to hit the target frequency and voltage range they've set for the 5800X3D, why not validate those chips at a lower frequency and voltage and sell it to people that want a fast, cheap, low power chip for gaming while making a profit instead of throwing it in the bin? Same as the 5700X vs 5800X. Makes perfect sense.