Evangelism:

Evangelism is greatly helped by believers living in community because it allows them to display their beliefs to the world.
Evangelism involves the gospel message being presented to the lost. However the lost think about much more than the gospel: they see you and how you talk (fidgety or bored), they know your lifestyle and whether you live it out, they also have a view of the Church (especially those in America), and they have other conceptions of Christianity that are messing with your message of the gospel.
So the more ways you can help people hear the message, the better. From experience I can assure you that when they see a godly family in action it helps tear away their misconceptions so that they hear the gospel clearly. Just imagine how much it would help for them to see the whole church loving each other and working together in a way that completely fits a coherent worldview. They might have seen churches on Sunday morning, but just think how great it would be to have them over to your house and to have so many godly families around you. They would get a clear picture of the Bride of Christ.
And that is just the evangelism side. The discipleship side is even better, and when the disciples work together it helps increase the evangelism. Christ’s Church really is like a growing snowball, a big, pure, white snowball rolling across white fields!