There's a lot of nay saying about first aid in the zombie preparation world. I don't know why. Sure, if one of those rotting shamblers bites you, you're done for. Do us all a favor and put the bullet in your own head.
But all kinds of things can go wrong in the zombie apocalypse. And chances are good there will be shortage of trained doctors. (I suspect many of them will be compromised in the opening stages as they are called to treat the infected.) So you need a basic knowledge of first aid.
Here we look at using the tourniquet. It's useful for many combat-type injuries. While I advocate cooperation among survivors for the long term benefit of all, I am not naive. Others will not take the long view, and chances are good you will encounter hostile human survivors.
Keep in mind the tourniquet is not a long term solution. It is intended to treat you immediately until someone can get you safe for better treatment.