they have you put it in the bag so if it is compressed, the change in altitude doesnt cause it to burst and possibly ruin equipment and other stuff. although the cabin is pressurized, there is a change. considerthefollowing.jpg

So... this pressure compression danger only started like ten years ago? Where was the pressure compression danger for the fisrt 100 years of flight? Or has the Earth's atmosphere or whatever changed in the last ten years? Or maybe aircraft have become less pressurized and more prone to sudden vacuums?
Or maybe they just want all your dangerous shit in one easy-to-examine clear pouch so as to keep the line moving?