Since there were no quad-core Broadwell laptops from business manufacturers (and only a handful in gaming lines), the jump is two generations ahead from Haswell.

The biggest incentive is the increased memory limit: reasonably compact machines with 32gb+ are now possible, and workstation-class machines can now go to 64gb. (Also, compared to the handful of Broadwell systems one might actually want to go that high on memory in, the move to DDR4 is starting to make bigger memory affordable.)

Compared to the Haswell generation, the speed bump of the new cores is also non-trivial even if not huge, and the increase in batter life and iGPU performance even more so.