Answer
In a nutshell, we must learn to walk before we begin to run.
It is true that in the everyday world, idealized cases rarely occur, but understanding the real world begins with understanding a simplified model, which may give us a pretty accurate answer. We then tweak our model for better understanding and agreement with the observed results.
Interestingly enough, there actually are idealized cases that appear in the real world, such as superfluid liquid helium that flows without any friction whatsoever, or superconducting currents that flow with zero electrical resistance.
