^they are useful for when your check engine light pops on. some cars have it automatically come on at predetermined mileage milestones (40k, 50k). in that case it takes you 2 min to reset it instead of paying your dealership to hook it to their computer than then 'discover' a bunch of issues that need fixing.

other times the problem could be minor, and you can fix it yourself:  my last three 'check engine lights' were an oxygen sensor, camshaft position sensor, and the brake indicator switch.  i fixed all three in my driveway for under $25 (new parts from ebay). diagnosis is free, and sometimes its a part you can change yourself.

other times some sensor glitch may throw a one-time error code, and your check engine light will be on (and you wont see other problems) until it gets reset.  if someones tech savvy, has a laptop and a car made after 1996, they owe it to themselves to get an ODB-II cable so theyre not totally at the mercy of their dealer/mechanic :)