Where Xdebug is particularly useful is in examining applications that are near completion. The documentation explains the available reports it can generate, which can give an excellent overview of where the main bottlenecks lie, and help you focus your effort on specific functions and routines. It also encourages you to take a more systematic approach to optimizing your application, rather than using educated guesses in an ad hoc manner.
You can also log Xdebug data to a file, which will allow you to gather profile information while your application is online. Sometimes, problems won’t show up until you’re running in a live environment; the impact to a database table which now contains many megabytes of data, for example, might make itself clear only once your application’s live and online.