Missions has a long history of promoting unity among Christians. The oldest Baptist denominations for example were organized to send missionaries. What was difficult for a single church to do, support and send long-term missionaries, was possible in partnership with other like-minded churches and Christians. Unfortunately, denominations, which I believe are capable of significant good in the cause of advancing the Kingdom through church planting and missions are often seen as divisive rather than unifying in their nature.
As a missionary, I have more than one ministry. I have a ministry that God has called me to among the unreached people of Japan--to plant the Gospel, churches and make disciples among them. In addition to this mission though, God has given me a second, equally urgent missions. That mission is to call Western churches and believers to unity in the cause of Christ among the Nations!
Very few churches are capable of sending missionaries themselves--there are thousands of churches throughout the USA and Canada that have less than 100 members. These churches are no less called to missions than churches of a 1000, that have million dollar budgets and designated missions pastors. In fact, from what I have found, the average person in a smaller church is much more interested in missions than a person in a larger church.
However, without thoughtful cooperation, it wouldn't be possible for a small church to intentionally send missionaries. They may be able to support a few parachurch missionaries that show up at their doors, and send their youth group to Mexico every few years, but that isn't the level of engagement in the advance of the Kingdom that God calls us to.
Denominations exist, and have existed primarily to plant churches and send missionaries. The Baptist General Conference, who I serve with, was formed from like-minded Swedish Baptist Churches with a desire to work together to send missionaries--in fact, it was the call, "53 Missionaries by 1953!" that became the rallying cry that unified the Baptist General Conference, Converge Worldwide as one denomination in the mid 1940's!
I have a ministry to diverse churches, inside and outside of my denomination. I have a message for them: we can do more together than we can do individually!
Even if a church is large and prosperous enough to send missionaries without cooperating with any other churches, is that really edifying to the body? I have been to a half dozen large churches that have a reputation (which is often indicative of reality) of not playing well with others. Rather than sending missionaries through their denomination, or even through parachurch organizations, they send them themselves. In effect, they are re-inventing the wheel in order to maintain control over their missionaries.
More than one of these churches is in decline, and visiting with the missions pastor I have heard that they are having a hard time being faithful to the commitments that they had already made. This to me is a tad ironic--when they were prosperous growing churches they stopped cooperating with others and took the entire burden (and blessing) of sending missionaries on themselves--and when the economy, or changing demorgraphics, or a retiring pastor affected the health of their church, they began to doubt the commitments they had made.
Had these churches instead continued to send their missionaries through a denomination, or a parachurch orgnaization, and insisted that they spread out the burden (and blessing) of their support to other churches and Christian individuals, the health of a single church wouldn't be putting the ministry of these missionaries in jeopardy.
Unity in missions promotes health in both small churches and large. I have the opportunity to cast this vision among the churches that I visit--that we are better together than we are individually.