In order to get one (or a couple of the achievements actually) you want survivors to get hurt and die sometimes.
SPOILER ALERT One of the secret achievements is to have 5 of your survivors die. The other is to heal survivors 25 times.
And yes Darque is right the danger level percentage when it is higher means the probability of your survivor dying is higher or being injured for a higher percentage of their health.
BUT if you have like 3 survivors with plenty of weapons that you do not need that you can spare for them you have a good chance of all 3 making it back with little damage. And if you sent them out for food (which is the most important thing) they should come back with plenty for you if you need them. You could heal them to start working on that achievement where you heal 25 survivors because that and alot of the accumulative achievements carry over from different playthroughs of the game.
I personally don't think it's worth it even though I send them out on every mission. I usually always have at least 3 people with their preferred weapons of choice. I will then send them all out for food with a 10% risk. They come back with just enough commercial MREs to heal the wounds that they aquired from getting the food. So it really doesn't seem that worth it IMO. --- Hold your fire! Hold your fire! It's only a squirrel. He's got something in his hand! -\o_o/-