Why do people add you as a friend if you don't know them?

Why do people add you as a friend if you don't know them? I went to my profile and I have all these friends; people that I don't know. What is the point of adding people?

I add people who have similar problems/interests to me.
I love making friends online.

If someone interests you, you can make them your friend to better follow whenever they enter anything. For example, maybe they say things that you really agree with, or possibly you like the way they think.

If you would rather approve people who request to be your friend, you can go to "edit my profile", then "friend management", and then click one of the two:

1) Let people add me as a friend automatically, or
2) Let me decide who gets added as a friend and who isn't a friend

I agree with BIMJOB

sometimes I do that by accident I click on the wrong thing