The Spanish needed a settlement between East Texas and the Rio Grande to consolidate their hold over the missions in East Texas and to keep those missions safe and supplied.
By the 1720s, the Spanish had built a number of missions in what is now East Texas. The missions were important to the Spanish both because they allowed the Spanish to try to convert Native Americans and because they gave Spain a greater claim to that area. By putting missions in the area, Spain was able to make sure that the region would belong to Spain and not to France. This was important at a time when Spain believed that it was building a lasting empire in North America.
However, the Indians of East Texas were not particularly cooperative. This meant that the missions would have a hard time remaining viable unless settlements were created between them and the more settled areas at the Rio Grande. It was for this reason that settlements were established in between, the most famous of which was the San Antonio de Valero mission, later to become famous as the Alamo.