Why shouldn't you inflate your life vest until you're outside the airplane?
Ethiopian Airlines Flight 961 was the flight that resulted in this addition to the safety briefing.

After being hijacked, the plane eventually crash-landed in the ocean. Many passengers died because they inflated their life jackets in the cabin, causing them to be trapped inside by the rising water.

Mayday/Air Crash Investigation S03E13 (Ocean Landing) covered the incident. The relevant clip is here (from 23 seconds in):