Day walkers, hikers, backpackers, cycle tourers, paddlers and travelers all need to take survival gear should they have an accident of find someone else who has an injury, but which one? Bivvy bag, survival bag, bothy bag, space blanket all have a place and each are specifically useful in different situations.
What you don't want to do, is carry too much weight or bulk, but still have confidence that 'should' you need it, it will keep you alive or at least enable you to extract yourself from whatever situation you find yourself in.
So here's a quick demonstration of what these survival items (Adventure Medical SOL Emergency Bivvy, Integral Designs Bothy Bags etc.) look like and suggestions as to how to you might use them: