Kind of a morbid topic, but an important one.
Unless you are still a kid or very young, unmarried, with very few asssets, we should all have one. If you are a parent, you should choose who you would like to raise your child/children in the event that the parents ae no longer around. If you have anything of value, it would be a good idea to have one so you can choose who should get what and avoid arguments over posessions later. It is also a good idea to have a living will - specifying what you would like to have done in case of a major accident. Do you want to be an organ donor? Do you want to be resesitated if your brain is not functioning?
At one time in my live, I could have ended up with about 15 kids. Thank goodness I wasn't needed to step in and take in someone else's kids. I think I was named in wills of three different families.