It is often argued that it would be more effective to fund nationals than send in foreigners to do missions work. The truth is that there is a place for both. The virtues of nationals making disciples are clear, but let's address the reasons that sending foreigners must always be an essential part of a healthy global missions strategy:
Consider that many who are living without access to the Gospel are in this condition precisely because there are no evangelical Christians in their community. Thus, their only hope of gaining access to the Gospel is for an outsider to come in and share the Truth of Jesus.
In some countries, Christians in one strata or Caste of society are not able to reach those in higher or more affluent spheres of the same society. For instance, an educated American outsider has a much better chance to reach a Brahman in India than does a lower caste Dalit.
3. "Immigrant Edge"
Some of the greatest missionaries in history were outsiders (consider Hudson Taylor, British missionary to China for 51 years). Just as the diligence of the Hudson Taylor allowed him to gain favor with many Chinese, outside missionaries often have the tenacity to study the nuances of language, culture, etiquette, to give them the edge as an effective disciple-maker.
Christ made it clear that we are to take the Gospel to all nations. Certainly there is a place for proper strategy and analysis, but let's not disobey the greatest command ever given for the sake of holding out for a better return on investment.