Why plant churches?
As a high schooler, I became obsessed with the importance of evangelism. But by “evangelism” I basically meant walking up to strangers and trying to get them to listen to me present my system for why they needed to convert. The whole focus of this was simply on making a “convert.” Scripture calls us to more.
Because Jesus says to:

Jesus told His disciples that he would build His Church on the reality of His person and work, and that “the gates of hell will not prevail against it” (Matt. 16:18). I used to hear that verse and picture the Church cowering behind its walls, safe from the scary onslaught of Satan. But that is not at all what Jesus is saying. Gates are indeed defensive structures, but what they are defending isn’t the Church; the gates are defending Hell, and Jesus is saying they won’t stand up to the onslaught of the Church! The Church is the powerful means of Christ’s defeat of Satan as it brings the redemptive reality of His gospel to bear on the creation.
We see more of how this works in the Great Commission where, just before ascending into heaven, Jesus tells his followers that He has been given all authority in the universe; therefore they are to “make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age” (Matt 28:19-20). He doesn’t just say to make converts, but disciples. Disciples are people being equipped for mission and obedience (observing His command). He links this discipleship to baptism: which is baptism into His Body, which is the Church. The promise to be with them to the end of the age underscores their participation in Him as His Corporate Body.
In Acts 14 we see Paul going out and doing this; visiting churches he has established and making sure there are elders installed to shepherd and care for the churches, equipping them for the work of mission they are called to.
Because it works:

In addition to Jesus’ command and the apostles’ example, we have the pragmatic reality that planting new churches is proven to be the most effective way to bring new people into a saving relationship with Christ. Missiologist Peter Wagner says, “planting churches is the single most effective methodology under heaven.” Statistically speaking, new churches draw an average of 60-80% of their new members from people with no previous connection to the Church (i.e. conversions) while established churches only draw 10-20% of their new members from people without prior connection to the Church (see graph below). This is because in established churches there are institutional-cultural rules that newcomers need time to learn; in new churches these rules don’t exist so it is faster for new residents, new generations and new people groups to learn to navigate the local church.
New churches and established churches should have a symbiotic relationship however (we want new churches to grow into healthy, established churches!): new churches can provide enthusiasm, passion and new people to the broader Church in an area or town while established churches provide resources, systems, training, maturity and funds to help out the new churches. With these relationships functioning well new and established churches are a powerful vehicle for expanding the kingdom and reaching the unchurched.