It's one way to emotionally connect with my husband... and it's also stress relief sometimes, and fun... It's my favorite thing in the world. Once when he was trying to talk about sex in front of our kids without them knowing, he referred to it as a "team building activity."
To me it is a form of intimacy that people in love share, but it can be anything to anyone else like it could be stress relief or some physical action or work out or something purely exciting with no emotions attached or even just a mundane daily activity which seems boring.
While it has become part of humanity's recreational lives in many ways, sex is nature's driving force behind procreation that causes living creatures to be attracted to the opposite sex, develop relationships and ultimately produce offspring.