The Basic Scriptural Reason
The basic reason for planting new churches is that this is the way the church grew in the book of Acts. One missionary journey after another resulted in new churches. The kingdom of Christ grew by the multiplication of churches......
Someone might argue "that was then, today we have lots of churches (buildings) - in fact more than can be filled by existing Christians. Shouldn't you spend the time filling the existing churches rather than planting new ones?"
Four other reasons
(1) Existing churches often lose touch with the world. It's one thing to say "fill the existing churches", it's another thing to do it. Many churches become inflexibly traditional and cease to be relevant to the culture around them. Their religious buildings put people off and their inflexible forms of worship are so distant from the culture that the Gospel is unable to cross the barrier. Why should a hoodie be expected to worship with a pipe organ (what's one of those?)? This doesn't have to be, but it often is. A new church has no traditional baggage and can adapt quickly to the culture.
(2) Existing churches can't change because of structural reasons. Existing churches may observe the great barrier between them and the world, but because of powerful internal institutional forces within may be incapable of adapting to the needs of the world. These forces may be due to internal politics such as the influence of long-standing families or other pressure groups within the church. So folk within an existing church may want to reach the lost, but find it impossible. A new church does not possess this baggage.
(3) Existing churches often grow indifferent towards the lost. Sad to say, but churches easily become inward looking for one reason or another, and cease to care for the world. Eventually some of them even justify their cold-heartedness with beliefs such as "it's the day of small things", "the world is so hard to the Gospel today", "the great commission was only for the apostles", "it's because we are being faithful that no-one is being converted" (they are right about this; faithful to man-made traditions), all the while neglecting the Master's words, "go..make disciples..." and "the fields are white unto harvest."
(4) Existing churches can easily become ineffective feeding centres rather than serving centres. In mathematics, the following is true.............
100 = 4x 25 = 2x50 = 10x10
.......but not necessarily in the kingdom of Christ. A church of 100 members may be far less effective than four churches of 25, two churches of 50 or ten churches of 10 members. Why? Because...
- the need to serve in a large church drops off as the numbers increase, "someone else will do it"
- ownership of ministry rises as top-down organisational commands give way to "this is the ministry God has called me to and I want to do it."
- relational organisation rather than organisational organisation. In a small church the family meet over coffee to decide who might look after children; in a large church a phone call takes place from an unknown stranger to another unknown 'church member'
- the friendliness to new people diminishes as a church grows and lost-in-the-crowd syndrome emerges; "Is this your first week?", "No, I've been here for seven years!" "Same here".
- sacrificial giving is required - or we'll be closing doors
- evangelism becomes a survival strategy rather than an optional activity
The plain fact of the matter is this: the Kingdom of Christ grows by church planting.