Suppose you want to register one SQL Server twice in Enterprise Manager. You want to test different registration properties from inside Enterprise Managerespecially permissions. In SQL Server 2000, if you try this, Enterprise Manager complains that you've already registered in the server.
The way around this is to use the Client Network Utility (choose Start | Programs | Microsoft SQL Server) to create an alias for your server. Click on the Alias tab and then click the Add button. Identify your SQL Server in the usual way. Then, register the alias in Enterprise Manager. At this point, the same SQL Server is registered twice, but with different registration properties (for example, with different User IDs and Passwords or as named pipes versus TCP/IP).
In SQL Server 2005, you do not need to resort to this convoluted way since SQL allows you to give a friendly name for each server which can be different from the actual server name.