Render is not an event. It is a method called to generate the output to send to the client. Render renders itself and its children.http://msdn.microsoft.com/en-us/library/ms178472%28v=vs.100%29.aspx
Because Browser understand only html markup So in Page Rendering All ASPX Controlls are Converted to HTML Controls Thats why we go for Page Rendering.
Browser understands an only html control that's why in page rendering we will convert the aspx controls into html controls.