Very important baby question!

What the HELL is a receiving blanket? I have tried everything I can think of to figure it out. Dictionaries only tell me that it is a lightweight blanket used � get this � to wrap up the baby. But everything I read, especially "must have" lists, mentions these things as though they are indispensable child-rearing tools. HELP! Please tell me why these things are so special and what makes them different from any other blanket.