Changing the oil is a regular part of car maintenance, doing so keeps the many components of the engine working efficiently. This alone is why the oil should be topped up, but there are other important reasons as well.
The oil is part of the car engine, it keeps it running smoothly and also helps to reduce the accumulation of varnish and carbon from collecting inside. A car engine can get very hot from the thousands of small explosions taking place every minute; oil helps pull the heat away from the combustion chamber essentially preventing the engine from blowing up, which is a good reason to keep the oil topped up.
No matter how good the oil is, eventually it degrades as the additives get used up. That’s when you should have your oil changed. You can tell when you need to change the oil by inserting a dipstick into the engine, checking the oil level and topping up if necessary.