difference between =" " and IS NOTHING

 The following code works as expected.  But when I change - If signInName = ""  (to) If signInName IS NOTHING - is does not work.  Why is this?
Dim signInName As String = HttpContext.Current.User.Identity.Name If signInName = "" Then Response.Write("NO SIGN IN NAME") Else Response.Write(signInName) End If
 
...