linkedin?

I’ve been treating linkedin requests as spam in my email inbox. I really don’t need any more email. But I’ve gradually become aware that some people I respect seem to be using this. The one thing I’ve heard is that it is used more by business people than academics. What do you know about this? Is it worth doing? What does it mean if someone I don’t know asks to “connect” with me on linkedin? What does it mean if someone I do know wants to connect? Is this something useful or just another annoyance?