Without it, our ancestors would still be swimming in the primitive oceans of Earth.
What were these simple organisms looking for? Were they just swimming because they could? Maybe they were looking for each other. Maybe they just stumbled on sex. But when they did, well, the evolutionary ball began to roll toward us.
The physical satisfaction that sex brings is another reason why it matters. What matters most, however, is that it deepens the connection, the bond people have with each other. Sex without love is, well, just sex. Sex with love is friendship on fire.
So sex has played a role in human lives since life began on Earth several billion years ago. And think of the role it played in your life. The odds against you being born, for example, was astronomical. Of the trillions of women who have become mothers since the human race began, one gave birth to you. Now think of the hundreds of thousands of sperm that compete with each other every time a man and a woman have sex. Only one joined the egg in your mother's womb to become you.
Another aspect of the physical importance of sex is that it allows the exchange of genetic traits that can be inherited by our children. Some say insanity is also inherited—we get it from our children!
Sex matters at the emotional and psychological levels because it's an expression of love. Relationships are less likely to be satisfying or to last if people are not physically attracted to each other. If that's the case, then physical affection, with or without sex, is probably absent as well, making it even more likely that the relationship will fail. And having many sexual partners will lessen the bond between any of those partners.
Promiscuity is the arrogant, philandering mindset of a savage. Faithfulness is the existential, responsible mindset of a civilized human being. Promiscuity is about sex. Faithfulness is about love.
Aging tends to diminish sexual desire and activity. Senior citizens, however, have less intense and longer lasting ways to nourish and maintain intimacy, such as deeper, more meaningful conversations, holding hands on a long walk in a park, and that wordless understanding that a couple acquire from being together for a long time.