Detergents are basic because that works wonders on greasy stuff. When oil polymerises it won't be susceptible to basic substances anymore but will react to acids. (Unlike acid and oils which don't really react with each other – think vinaigrette separating in the fridge.)

Washing a cast iron pan with detergent will clean it from unpolymerised oil.

Cooking e.g. tomato based sauces in your cast iron pan will strip it of the polymerised coating (might impart flavour too).

Cleaning kitchen tiles near your stove is sometimes easier with acidic cleaning solutions as well. Just be careful with the caulking which will brittle over time from using acids.