You can be fined in some places if you do not have one. Additionally, your car can be impounded, and you can be held up by authorities until they grow tired.
Or you can simply pay $15-20 and get an IDP. All you need are two passport photos, a valid U.S.-issued drivers license, and the application, which you can download from AAA or AATA.
AATA IDL requires you to download and mail in an application form with the photos and copies of everything, but you can walk in to your local AAA office and you’ll leave with your IDL in hand.
The IDL has to be renewed annually and must be presented along with your driver’s license to be valid. It’s only good for vehicles in the same class in which your U.S. drivers license is issued.