Why is a better question? The answer is very easy to understand.
Years back appliance plugs blades were all the same. But unfortunately, they could be plugged in any which way. This is really bad. Anyone that has ever done any electrical work knows that the neutral, hot and ground has to be installed the proper way on the receptacle. And of course, that would also mean the plug going into that outlet should also be inserted the correct way.
The hot and neutral should carry all the way through into whatever you’re powering. Let’s just look at the metal casing of an appliance or better yet the metal casing of a drill. If you were holding the metal drill and drilling into a metal fence post, well lets just say you my friend can have a shocking experience.
You would wish the metal part of that drill was grounded and not connected to the hot leg. Either one will work for sure, but not safe. This applies to anything you plug in.
Think of a lamp, everything works as it should, but then you turn the lamp on while in the sink or something like that.
Now let’s not misunderstand a polarized plug and grounded plug. Just because it may be polarized does not mean it’s grounded.
A short story I’ll never forget
Years ago some guy just bought a house and while his family was in the backyard playing he decided to trim the hedges with an electric hedge trimmer. All was well until he held the trimmer in one hand while he rested the other hand on the metal fence post. Even though it was only 120 volts, he was the ground between the trimmer and the fence post and he could not let go and unfortunately died right there in his backyard.