The only three reasons I can think of are the following:
1. You are paranoid that somebody will hack into the Department of Motor Vehicles and use your licence plate number to find your address and then go steal your car.
2. You are using stolen licence plates and don’t want to get caught by the authorities.
3. That’s just what other people do and so there must be a reason for you to also do it.
Don’t we all drive our cars on the street where our licence plates are freely visible to an infinite number of thugs, hoodlums, reprobate swine, and fraudulent skinflints?
pasghetticode posted a link to a great article on this topic from 2012. Who knew!?