Understanding people can always help with coexistence, but a good relationship goes beyond just good coexistence. If you know someone's personality, it is easier to understand what someone means, even if they use any sort of unusual signals. Also, you get along better with a person by knowing what they like and dislike and using it to improve the relationship. Personality traits are important to know because by knowing them, you can avoid misunderstandings. Many people take weird behaviors by others as a personal attack (he didn't say "bye" when he left), but many times, it doesn't meant that the unusual behavior means that person doesn't like you.