Contrary to what many people believe and what some answers here say, modern 100% synthetic oils of good quality do not degrade with use, or at least not noticeably in the periods we are talking of for a moderate common car use. The oil molecules as such do not loose their lubricating properties, what happens is that the oil gets dirty, unavoidably. The perfect oil filter does not exist, if it did you could run for many tens of thousands of kilometers just replacing the filter but not the oil itself.
The reason why the perfect oil filter can not exist is that several of the oil molecules are rather large, larger than many of the possible dirt molecules, so when you have to make a filter which must allow all of the oil molecules to pass through, unavoidably it will also let dirt particles go through. Good modern oils include detergent molecules which stick to dirt molecules making them larger and therefore getting them stuck in the filter, but they can not do this to all of the possible dirt molecules, some unavoidably pass through the filter and remain in suspension in the oil.
However it is important to understand that most of the dirt remains stuck in the filter, much more than in the oil itself, so replacing the filter is vital, some people sometimes just replace the oil without replacing the filter and they think they are taking good care of the engine because they put new fresh oil in it. That’s a big mistake, always replace the filter, it’s where most of the dirt is!
One last comment, oil and filter replacement are most critical for a new car, when the moving parts still need to wear out and get a close and tight adjustment. Forget the manufacturer recommendations, they always try to extend the official maintenance periods in order to reduce their official maintenance costs, the first oil and filter replacement should not be done any later than after 6,000 or at most 10,000 kms. After that if you use a good filter and oil you may extend the period.