There are more fluids in your car than just oil. There's transmission fluid, brake fluid, power steering fluid, coolant and more. You used to be able to check most of these by opening the hood and checking the dipsticks or eyeballing the reservoirs. Actually, you still could, but who has time for that? After all, you want to get home so you can rewatch "Lost" from start to finish and really pay attention this time.
Good news for you. Sensors can easily track the levels of these fluids and let you know when they're low or in need of a change.