The easy answer is that it is what every follower of Jesus is called to do. Read the Bible and you won't be able to disconnect a follower of Jesus from being a part of a community of believers. But if you need more than that try these reasons:
1. Understand the Bible better through group discussion as you apply the Bible to your own personal situations.
2. Develop close relationships with others who will walk beside you in your journey as a Christ follower.
3. Find answers to the needs in your life through group prayer, discussion, and study.
4. Get support in times of crisis or major changes from people who really care for you.
5. A place to serve your friends and others who don't know Christ in a nonchurch setting.
6. A place to move from being a spectator to participant by using your gifts and talents to serve others.