Emergencies are unexpected and usually dangerous situations that require immediate action. It pays to be well prepared in case they happen, though by far the best option is to try and take as many precautions as you can do prevent them from happening in the first place.
There is emergency equipment to survive nights, like emergency blankets and bivouac packs. As opposed to this, if you plan to stay overnight on winter trips (e.g. in the Alps) you don't just want to ...