A bit of background on the Windows rolling down
For the longest time , I also thought it was to release the air pressure so that the Windows "don't break" from the impact. There is a bit of truth to that and here is how it breaks down as far as I understand.
Frameless Windows lack the positive contact to the seal upon closing. Have you ever noticed, if you grab the edge of the glass to try and close the door (which you should never do) the window flexes . Now imagine closing the door without it rolling down, the surface area of the door and glass will push a lot of air inside the car increasing the pressure. Its like swinging a big piece of cardboard through the air . The faster you swing it the harder it is to move all that air trapped in front of it. The same with your door, the pressure will try to escape and push against the window at the point of contact. At that point two things would happen .
1. The door would be slightly harder to close/require more effort .
2. You wouldn't get a good seal between window and seal . Because of the "burp" of air , the window would contact twice and only in one direction (perpendicular to the seal) and that isn't the best type of seal .
Thus .... It leads us to the window rolling down :
1 it lets built up pressure quickly escape so that the effort required to close the door is always the same
2 it prevents the little burp and weak seal contact
3 when the window finally rolls back up, it creates a positive seal as it pushes up and into the seal . The final result is one by where the seal isn't only pressed up against the glass but also ridding over the edge slightly . This effect doesn't have to be great , meaning you won't see it from the outside
Anywho ... I hope that helps
I will introduce myself soon , my appolagies for not doing it before this post . I felt compelled to answer this question