There are probably multiple reasons that churches should plant churches.  However, there are three specific reasons that come to mind.

1. It’s biblical.  We see in Scripture that as the church expanded it multiplied.  It multiplied by establishing churches in surrounding communities as well.(Honestly- if this is reason number one do we even need more than that?)
2. It’s challenging.  As people from one church step out to help plant a new church we find that they often leave leadership vacancies in the sending church.  This forces less active members to get more involved and step into those roles.
3. It’s evangelistic.  Statistics tell us that a church has some of its most fruitful evangelistic work within its first couple years of life.  This is most often the case because of the great urgency that is felt by those charter members.