Why would you want to start in safe mode?

Well, for trouble shooting purposes, or routine maintenance, you should start your Mac in safe mode. it’s simple.

Turn your computer on, and immediately after  (not before) you hear the Mac startup tone, press the shift key, and leave it pressed until you see the Apple logo. You will see a progress bar during startup, accompanied by the spinning gear.  This is a good sign, since it means that your computer is performing a directory check as part of Safe Mode.