According to Abba: The Official Photo Book, published to mark 40 years since they won Eurovision with Waterloo, the band’s style was influenced in part by laws that allowed the cost of outfits to be deducted against tax – so long as the costumes were so outrageous they could not possibly be worn on the street.
Why does this tax exists? For taxing ads? Like for promoting a clothes brand?
I think you are missing the point. You can declare products you buy as a business expense, as long as they are used mostly for business purposes. In Swedish law, you can’t declare clothing as a business expense because you will wear those clothes in you day to day life. The only exception is clothes that would look silly in day to day, like a clown suit, or mechanic overalls.