It seems like it would be much quicker and easier to just have a piece of plastic or something cut at a 76 degree angle that they can place on the laptop and fold the screen up to.

Could be that the demo OS reports some metric on how often the laptops are set to 76deg and how often customers move it. Probably a whole ton of usages of the sensor and if it's price comparable to the old close sensor they used to use it would be easy to justify.

I've heard employees use the measurements app in their iPhones sometimes to adjust in the mornings, but having a sensor in the laptop lid seems like a much easier way to do it and you don't need to carry anything with you.

It would not, since you don't want to carry a piece of plastic all day long to set the angle correctly. Most people just use their phones to check the angle though.