Christ commanded churches to bring the Gospel to every creature. God commissions missionaries to leave their churches and preach the Gospel.
Not every church has the staff resources to help their church’s missionaries with representation before foreign and domestic governments. A church may become its own agency or enlist a missions agency to assist their people in their Great Commission work.
A mission agency handles the clerical, legal, and technical matters of receiving support so the missionary can focus on their commission, and their church can focus on their command. Thereby, the world can receive the Gospel as Jesus commanded.