I tried reading the MSDN article on markup extensions, but I can’t find out what they are (the article discusses what they do).
I cannot find a clear explanation of why we need markup extensions. If we can access a control object directly, why would we need a markup extension to access a binding object?
Do we need markup extensions so XAML is aware of the code behind (otherwise there is no way to get access any of the built in classes)? But then how can we access all the control types?
Markup extensions are not about access but extending functionality of markup (as the name implies) by doing whatever you want them to, like creating associations (Binding, x:Reference) or getting the type of a class (x:Type).
They can be used for just about anything, they are only necessary where the markup does not suffice on its own.