SqlParameterCollection: Which to use Add Method or AddWithValue Method? Differences?

I was wondering what the proper usage for both methods.  They seem to work regardless but which one is the preferred method?  I saw this brief explanation but it didnt make sense to me in the example given.  http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlparametercollection.addwithvalue(VS.80).aspx

Thoughts?

Here is my sample

 1. Add method
         ' Get the Primary Key Id
        Dim refId As Integer = FormViewClientCode.DataKey.Value
        ' Get the Client Id and Client Name to be edited
        Dim txtBox1 As TextBox = DirectCast(FormViewClientCode.FindControl("txtBxClientCodeId") ...