All this “overlap“ people like to complain and wonder about here in the forums is a best practice for businesses of any kind. You offer a “ladder“ of products, with nice easy steps between them. So, yes, one reason is to have something to offer at different price points.
Even more importantly, a MBA 15" and a MBP 14" serve different purposes and different customers. Case in point: I own a MBP 14" (M1 Pro) right now and I'm in the market for the MBA 15". How and why? Because I realized I want a more powerful desktop than the MBP 14" (I used it in clamshell mode most of the time) and I want a more portable laptop than the MBP 14".
Therefore, I've already bought a refurbished M2 Max Mac Studio. That machine is awesome. I expected it to be quieter than the MBP, but I didn't expect it to be silent even under continued load. Amazing little box. I'm glad I went for it.
I could have gotten a MBP with the same specs, but a) they are louder under load, b) the thermals are generally much better with the Mac Studio and c) less pieces that can fail. I was getting concerned about the health of the components of my MBP when it ran warm so often and was being used all the time for everything. Elements like battery, keyboard, track pad, hinge, display are separate or don't exist in the first place with my Mac Studio.
Now I'll keep my MBP 14" around until the M3 MBA 15" shows up and then I will exchange it for that one. Sure, the MBA looks worse on paper. But it will be better in real life for me and my use cases.
So, yes, I “know“ what 120hz is and I still don't care enough. I also don't need the ports on my secondary computer and if it has to stand in for my Mac Studio at some point, I'll just connect it via USB-C to my Dell monitor and I have all the ports I need. Also the M3 has plenty of power for a lot of tasks. The difference in display resolution between MBA and MBP is irrelevant for me. And so on.
I'm glad Apple finally makes a 15" MBA!