I speculate they gave themselves a lot of thermal engineering margin to bump up TDP with the M-series MBP design (or perhaps they underestimated how good the M-series chips were going to be) The battery being at the TSA limit of 100Wh is quite nice as well. Another benefit is that it now differentiates the "Pro" line from the rest of the laptop lineup quite significantly. For most people the Air has enough power now and its plenty thin and light. The pro line is for "true" pros with actually intense workflows.

I'm a dev and the MBP line is definitely overkill for me. The 15" MBA handles everything I can throw at it.