This is a method of bracing the weapon allowing for a [somewhat] stable firing position in lieu of being to use the ground, wall, or other platform. While not preferable it is quick and often one cannot always find a good place to bipod, etc. IE: Such as where one is on a decline but firing at a higher or equal elevation or firing over hedge (as is shown in one of the photos) or other things that can obscure a gunners vision. Unlike in fiction, it is very inaccurate to fire machine guns from the hip, shoulder, or unsupported. This method is still occasionally used today (though once again as a last resort and often with a different position) and is used for sniper rifles as well. As for is it uncomfortable and hazardous... it is not comfortable. One needs to ensure they are not too close the the muzzle or the flash can burn, the barrel can heat up rapidly in cases of sustained fire, and just remember that hearing protection was largely absent in combat.