From Christopher Wright's The Mission of God's People:
[Matthew 9:36-38] is the only other time in the Gospels [besides the Lord's Prayer] when Jesus explicitly tells the disciples what to pray. And it is unmistakably missional - in context and content.
Why...do we use the 'Our Father' prayer so regularly in Christian liturgy, and this 'other Lord's Prayer' so spasmodically? What might have been the story of Christian mission if this prayer had become the one we memorized and repeated (and meant) down through the centuries? Of course, it's a dangerous prayer to pray. It tends to become self-answering, as the disciples found. For if they did do as Jesus told them, the very next thing that happens is that they become the answer to their own prayer as Jesus sends them out (Matthew 10).