gives you something to do when you're bored or have spare time
you're less likely to feel lonely or to become withdrawn or isolated, it might contribute to a better mood and a more positive outlook on life, better mental health and well being
gives you experience of dealing with lots of different types of people, people from different genders, ages, races, socioeconomic backgrounds, etc etc
improves your communication skills and general conversation skills, may even encourage you to broaden your knowledge in certain areas depending on who you hang around with
makes you more confident around people
has the potential to give rise to significant long term friendships and/or relationships