Because the gospel brings glory to God by spreading to new places. This is Paul’s point in Romans 15 and why he leaves behind fruitful ministry in Jerusalem, Illyricum, and everywhere in between. God is glorified by reaching people in the far-flung corners of the earth, not just by the sheer number of believers themselves. It’s how the gospel works, preach the gospel and your people will want to share the gospel, not just in their own neighbourhood, but to the ends of the earth. If that’s not happening there’s something wrong.
Because the local church is God’s mission agency for the world. Even the messed up Corinthian church was a mission agency 1, so was the Roman church 2. God’s strategy for reaching the globe is not local churches paying mission agencies to do it for them, but local churches rolling up their sleeves, sending workers and support 3. In short, if our local churches are not involved in some way we are being unfaithful to what we’re called to be. Local churches are never in the New Testament told to start youth ministries, but they are told to send money, workers, and support other gospel churches.
Because the missionary mindset is the mind of every Christian. Being concerned for global mission is not a freakish minority interest for only the keenest, rather it’s normal Christianity for every believer. In Philippians 2 Paul is persuaded that self-sacrificial concern for the eternal well-being of others is the mindset given to every believer. It was the Philippian church sharing that mindset which made them send Epaphroditus, give to Paul and stand with him even as he was imprisoned. This doesn’t mean everyone must be sent, but it does mean that everyone should be involved, be that in praying, giving or going because that’s the ordinary Christian life.