Why do they actually have sex scenes in films?

This might seem like a stupid question, but I'm being deadly serious.
What is the point in a film showing a sex scene(s)? 98% of the time,
the scene isn't necessary and doesn't add to the story.

I can only think of a small handful of films that actually
needed/benefitted from the sex scene, e.g. Terminator, Oldboy, etc...
But for the majority of the time, they don't, so why do they use them?
I'm guessing to get more viewers but is there any other reason?