Much moreso than humans, animals are attracted to aromas of other animals. When female animals are in 'heat' (sexually ready to reproduce) they give off a kind of 'musk' smell that is very apparent to other animals of that species.
Compared to animals, humans have very weak sense of smell, and males sense of smell is weaker than females. Women do have more acute sense of smell than do men and studies have shown that women do choose men based partially on how they smell.
Therefore, it's better for men to smell better than women, since to us it's not that apparent. Unless it's cologne. But the studies show that although women pay attention to whether the man is clean and smells nice, part of what she can detect is his genetic code, transmitted to her through his natural aroma.
On some Greek islands, the tradition is for a man to dance all evening with a handkerchief in his armpit (no, they don't use deodorant). At the end of th evening, he gives that sopping handkerchief to the girl he's interested in as a kind of courting ritual. She can then smell his natural pheromones.