It is probably not a trench, but just a plateforme.
When you want to camp on the snow, you can not just set your tent on top of the snow. It's not hard enough, even if it's crusty and not to thick ; the tent would fly away at first wind. (plus, your body weight would form a hole in the snow under the tent : and sleeping in a snow hole is quite cooler than sleeping on top of an even, solid layer of packed snow - or on frozen ground.)
Plus, they probably wished to have a horizontale bed, not a steep one !