When their faith is weak, they can seek the encouragement of others. When their faith is strong, they can help the weak. We all need to give and to receive fellowship. Our weekly meetings demonstrate that fellowship is important to us. Fellowship is sharing lives, sharing emotions, bearing one another's burdens, encouraging one another and helping those in need.
Small groups are a good place to fellowship too.
We get to know people a little better and feel a little safer with them. When I list service as a separate category here, I am emphasizing physical service, not the service of teaching. The early church gave physical help, sharing their possessions with the needy, collecting offerings for the hungry. Those who isolate themselves fall short in this aspect of Christianity. The Christian community also serves the world around us.