Save money on policing, HMRC, imprisonment. Also remove the criminal element from illegal productions etc. This matter should be a personal choice.
Pornography is very much a personal view. An old friend of mine used to say that the erotic ends where pubic hairs begin. Our national galleries are full of nudes, nude studies. Explicit and graphic depictions of consensual sex are probably preferable to the graphic depictions of violence we are assaulted with in war, horror and crime movies.