Why? Because here be dragons. And sometimes clearing your settings may be the best way to fix a weird problem. Keep in mind that settings are not cleared by uninstalling or installing new viewers
If clearing settings solves your problem, then you might want to create a backup of your settings once you have restored them as you like them - and assuming that everything works. Click here for instructions.
Edit - Preferences - General Tab - Reset All Settings (located at the bottom)
This can be done both on the Login Page and within the viewer after logging in.
NOTE: This does not wipe all settings; per-account settings are not reset. So it may not be effective enough to solve issues you might have. The method below is more complete.
This is a bit more complex but it ensures that everything is wiped.
Log out of Phoenix. Then navigate to the folder that stores the application data for second life - if you do not see this folder, you may need to show hidden folders.
NOTE: For Firestorm, refer to this page.