I guess in relationships it’s nice to have sex, as it creates and maintains an intimate bond. Good relationships are often made when the individuals are best friends, so I feel that sex takes the best friend relationship, into a relationship.
Sex is obviously not everything and like you said, many are able to go without it. Sex is clearly to allow us to reproduce, so it is an innate desire in the majority of the population. Perhaps over the years some people have evolved to have little or none of these desires, hence why some people identify as asexual.