FTP retrieval is also limited by the depth parameter. By default, Wget will create a local directory tree, corresponding to the one found on the remote server. Recursive retrieving can find a number of applications, the most important of which is mirroring. It is also useful for WWW presentations, and any other opportunities where slow network connections should be bypassed by storing the files locally. Careless retrieving can also fill your file system unctrollably, which can grind the machine to a halt.
Recursive retrieval is a good thing when used properly.
Please take all precautions not to wreak havoc through carelessness.