C# asp.net Why is there a difference between ClientID and UniqueID?

I know ClientID is used for javascript and UniqueId for server side and that ClientID uses an underscore (_) and UniqueId uses a dollar sign ($) in asp.net 2.0. But what I don’t get is why use two different id’s. Why isn’t possible to just OR use the underscore OR use the dollar sign in both: server and client side. Can someone explain this?