Why Church Planting?
Before answering this question, it is far more important to answer the question...

What is the Church?
Mark Driscoll and Gerry Breshear give an excellent definition of church in their book Vintage Churchprimarily based on Acts 2:42-47:
The local church is a community of regenerated believers who confess Jesus Christ as Lord. In obedience to Scripture they organize under qualified leadership, gather regularly for preaching and worship, observe the biblical sacraments of baptism and communion, are unified by the Spirit, are disciplined for holiness, and scatter to fulfill the great commandment and the great commission as missionaries to the world for God's glory and their joy. (from Vintage Church, © 2008, p.40)

Because that is a little long, the definition of the church our church plants use is:

The redeemed people of God living on mission together under the headship of Jesus to live out the implications of the Gospel.
So again, why plant churches?
For years, statistics have shown (and continue to show) that church planting is one of the most effective ways to invite people to step into the life-giving waters of Jesus Christ. Church is not to be just a one-day-a-week event, but a community of believers loving their towns and cities into a life-changing relationship with Jesus. Therefore, to continue to advance God’s Kingdom on earth, new churches must continue to be started while existing churches continue to mature and reproduce.

Dr. Tim Keller states this very plainly:
The vigorous, continual planting of new congregations is the single most crucial strategy for (1) the numerical growth of the body of Christ in a city and (2) the continual corporate renewal and revival of the existing churches in a city. Nothing else—not crusades, outreach programs, parachurch ministries, growing mega- churches, congregational consulting, nor church renewal processes—will have the consistent impact of dynamic, extensive church planting. (from "Why Plant Churches" ©2002) 
Church planting is not supposed to be a reaction to existing churches, rather it is to be a continuation of the Church. Just as an emotionally-healthy family desires to reproduce itself through kids, local churches must continue to find ways to reproduce themselves so that the life-changing invitation to step into the river of Jesus can continue to be given to all people in all contexts.
Quotes
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