I think they are to mainly to relieve pressure on the feed lips - note they push the rounds down a bit. I had another polymag - Keltec I think - that I left loaded for a short time; it expanded so much that when I shoved it in, it got stuck halfway in my magwell. After I yanked it out with pliers, it went straight in the trash.
I have never had that issue with a Magpul, but then again I keep the "dust covers" on.
Anyone know why the mil banned PMags? Is it all Magpul or just the
mags?