This is a friendly way to suggest what someone should do. It means "you should ___" For example, if you and a friend are waiting at a concert to meet someone but can't find him, you can suggest:
Why don't you wait here and I'll walk around and look for him?
It might be confusing, because "why don't you" may sound like you're criticizing the listener. But this isn't how it sounds to English speakers. This phrase sounds friendly, light, and casual.