If you have a car that throws consistent "Check Engine Light(CEL)" warnings or a car that has an intermittent CEL, you will want to have one of these on hand. Also, car dealers are not the most trustworthy so if you take your car to the dealership for repairs, you will want to have some idea of what is going on before you talk to them as 9 of 10 will lead you astray without pause or guilt. Also, the dealer will be charging you up to 2 hours labor just for hooking up a similar piece of equipment to your car to see what the computer tells them, then they stack on another price to check it out manually.