What's the purpose of the Global Assembly Cache (GAC)?

Question

I have registered a DLL in the GAC. I can see the DLL in the path: c:\windows\asssembly; I suppose this means the DLL registerd correctly.

However, if i have to use the DLL in my application, I still need to click "Add Reference" and add from the above path.

So, what's the use of the GAC if I still need to add the reference again?