When a Web server application finishes with a client request, it constructs a page of HTML code or other MIME content, and passes it back (via the server) to the client for display. The way the response is sent may differ based on the type of program. 

When a DLL finishes, it passes the HTML page and any response information directly back to the server, which passes them back to the client. Creating a Web server application as a DLL reduces system load and resource use by reducing the number of processes and disk accesses necessary to service an individual request.