If one just uses the FS/PH viewer, they may never need the Debug Settings. However, knowing about these settings will allow you to customize your experience. A good example of their use is in Camera Position Tips.
The Wiki for Firestorm-Phoenix has a list of the Debug Settings for those viewers. You will find the lists here: Phoenix Debug Settings – Alphabetical Listing and Firestorm Debug Settings – Alphabetical Listing. They are not clear which list is for which viewer. Hopefully I have it right.
Also, the lists are incomplete. Settings like; FocusOffsetRearView and CameraOffsetRearView are in the viewer but not the lists on the wiki. You can find the rest of the list in the SL Wiki here: Second Life Debug Settings.