Simpler than that I think - when do you turn off the screen or sleep? Because it isn't fully closed, but you want to be able to 'privacy-duck' the screen a bit before that, and having a sensor rather than just a fixed angle switch makes it software defined and something they can update.
If you approach something metalic to the top of the base in the left side of most macbooks you can feel where the magnet is. They either have both systems or maybe they switched this recently.
Presence of a magnet doesn't imply presence of a reed switch - are you sure that's not just to give it some resistance to opening for example? Or angle sensing could be implemented with a magnet and Hall effect sensor.
More likely a hall effect sensor, which is solid state and a lot smaller. And yes, older MacBooks had something like that, as evidenced by the fact you could put them to sleep by holding a magnet in the right place (just to the left of the trackpad IIRC in the models I'm familiar with)
Nice one ! Curious since I know almost nothing about HW - do magnets screw with computer HW otherwise ? I would guess no since we don't use HDD anymore but not sure.
As far as I know, even HDDs were pretty resilient to magnets when in their enclosures. I once took a large magnet meant for holding together concrete forms, one strong enough that it stuck to a ferrous surface it could probably support my weight, and stuck it to a hard drive for a full year to see if it'd break. The drive, as well as all of the data on it, were fine.
When I ran a MacBook Pro in closed clamshell mode and put another laptop on top of it, it went to sleep. Must be a weight sensor in there as well. (/s)
It's one sensor in both cases, and in the latter case you can do so much more: change the thresholds in an update, detect when the lid is in the process of closing, apply hysteresis (on a simple switch, there's an angle where vibration could cause it to bounce between reading open and closed, but with an angle sensor you can use different thresholds for detecting and open and closing state change).
But most of all...you don't have to commit to a behavior early in the design process by molding the switch in exactly the right spot. If the threshold you initially pick isn't perfect, it's much easier to change a line of code than the tooling at the manufacturing plant.
Why use two sensors when one will do? If you already have an angle sensor, it makes sense to get rid of the reed switch and reduce your production costs.
