What is importance of a polarity test?

HERE'S AN EXAMPLE:

IF THE POLARITY IS BACKWARDS AT A LIGHT FIXTURE THEN THE HOT (120VOLTS) WILL BE THE OUTSIDE OF THE LIGHT BULB SCREW SHELL AND THE MIDDLE PRONG INSIDE THE SOCKET WILL BE THE NEUTRAL -- THEREFORE WHEN YOU CHANGE THE BURNT OUT LIGHT BULB YOU WILL GET SHOCKED BECAUSE THE SCREW SHELL PART IS HOT!! IF WIRED CORRECTLY YOU WILL NOT GET SHOCKED BECAUSE THE HOT (120VOLTS) WILL BE INSIDE THE SCREW SHELL ON THE PRONG AND YOU WON'T BE TOUCHING THAT PART WHEN YOU CHANGE THE LIGHT BULB!!