Short-term missions may well show us that God wants us in fulltime missions. We can act as a liaison between missionaries and the home church. We can be used of God to lead people to Christ, which has eternal consequences. Financially, our whole trip will probably cost less than support for a full time Christian missionary for the same duration. Your trip may encourage other lay people to go.
Missionaries are good at recruiting missionaries.
When you cannot go personally, you will be more likely to support others going. We have a unique opportunity to experience the oneness of the church universal. Short-term missions is usually PowerAid for marriage. Short-termers can be a great asset to fulltime missionaries in the field. They also generate prayer and other support of fulltime missionaries.