It's not always possible to take a tent with you, or you're just going on a trek, a hike, or to the rocks with the idea of spending the night somewhere.... Then our "Žďársky bag" will certainly come in handy, which will protect you sufficiently if the weather doesn't work out and it rains, blows, or snows.
You'll also appreciate the benefits and philosophy of the bivvy bag in the mountains, when you don't want to lug a much heavier tent with you over the forest range. You'll enjoy more space in your tent, but it's like comparing soccer cleats to paint boots. Both are just suited for something else. So, in a simplistic way, a zhadar can be thought of as a good compromise between being fully at the mercy of nature when sleeping under a tent and a comfortable night in a canvas house.
In addition, its functions of protection from cold wind should be mentioned. You may experience an unexpected change in the weather or simply get lost and have to somehow survive the night on a rubble patch huddled behind a boulder without a sleeping bag. A lightweight jerkin can save your life, no exaggeration.