What is the use of XML comments in C# than signal line and multiple line comments.
i.Single line
Eg:
//This is a Single line comment
ii. Multiple line (/* *
From XML Documentation Comments (C# Programming Guide):
When you compile with the /doc option, the compiler will search for all XML tags in the source code and create an XML documentation file.
Also XML comments used by Visual Studio for IntelliSense:
///
/// This class performs an important function.
///
public class MyClass{}
Will give you nice hints when you are typing code or hovering cursor over member which has xml comments:

NOTE: Usually you should add xml comments only to publicly visible types or members. If member is internal or private, then it's good, but not necessary. There is nice tool GhostDoc (available as extension to Visual Studio) which can generate XML comments from type or member name. It's nice to check if you have good naming - if generated comment is not clear, then you should improve name of member.
I also suggest use simple (non-xml) comments as little, as possible. Because comment is a form of code duplication - it duplicates information which you already have in your code. And here is two problems:
Good comments should describe why you writing code instead of duplicating what code is doing.