What is the point of a double-sided poster?

You can flip it if one side gets dirty or put it on a transparent wall.

For putting in the window of an establishment: visible from inside and outside.

Yeah…but on the other side it’s a mirror image. It’s illegible.

Probably because you have a movie poster that is meant to be put up in a cinema light box, printing on both sides means you get a much richer better quality image while a single sided print looks washed out.