Oiling a cast iron skillet after washing and cleaning it helps to protect it from rusting.
Certain types of food can weaken the seasoning on a cast iron skillet and make it susceptible to rusting.
Adding a thin layer of oil to the skillet, inside and out, helps maintain the protective coating.
It is not necessary to go through the full seasoning process every time you oil a cast iron skillet.
While it may be possible to skip oiling if you have a well-seasoned skillet, it is still recommended to do so to maintain the protective coating.