How does Visual Studio/intellisense know what to do with a variable declared as var even if you don't include the necessary using declaration at the top?

For example, I have class MyDomainObject defined in a different namespace If I don't declare using TheOtherNameSpace; in the file the following code won't compile:

private void Foo()
{
   MyDomainObject myObj = new MyDomainObject(); 
   // Doesn't know what this class is
}

But if I use var

var myObj = new MyDomainObject();

This will compile, and intellisense knows exactly what I can with it.

So how the heck does it know what the type is without the using?

(And as an aside, if it knows without the using, why do we need usings at all?!)