ASP.NET shared functions

Hello to everyone,

1. When is it usefull to use shared functions in ASP.NET?

2.What happens when multi users use this functions at the same time?

here is an example

http://www.asp.net/learn/ajax-videos/video-286.aspx
Public Shared Function GetPostBackControl(ByVal thePage As Page) As Control
        Dim myControl As Control = Nothing
        Dim ctrlName As String = thePage.Request.Params.Get("__EVENTTARGET")

        If ((ctrlName IsNot Nothing) Go to the complete details ...