When we have sex, we tend to have a lot of our bodies touching, because it feels so good (so, so good!).
But when filming sex, our customers want to see what feels good, and that can mean some body-separation is necessary. The classic example is for penis-in-vagina sex; in most sex positions, his hips cover up the actual penis going in and out of the vagina - you guys can feel it for sure, but no one else can actually see it.
For this reason, sometimes on shoots our Shoot Producer will ask you to cheat the position (more info). Cheating is a common technique in all sorts of performance, not just porn.
A practical example might be not penetrating her vagina so deeply, or penetrating her vagina from an angle, so the camera can see the details of the penis going in and out of the vagina (and thus, so our customers can). When customers can see exactly what's happening, they can relate to it better, and feel more like they are there, in the scene with the female model.