If you drive with expired tags, police officers have just cause to pull you over and give you a citation. If you've gone longer than 6 months without renewing your tags, you face more severe penalties such as hefty fines and even jail time.
Every year you are required to renew your car's registration with your state's DMV. Tags are proof that you have registered your vehicle and paid the tax.
You're legally required to register your car as well as have a valid driver's license and active car insurance policy to drive on the roads.
Since having expired tags is considered a non-moving violation, you can be issued a citation even if it's parked and you aren't driving.
In some states, if you repeatedly drive without current registration tags, you're liable for paying heftier fines and even face up to 60 days in jail. This is a severe punishment that doesn't apply in all states, but it's a potential consequence of driving with expired tags.
Failure to pay your fines is a crime. If you don't pay your citation fees to the state and renew your vehicle tags, you're subject to a warrant for your arrest.