Because the eunuch was traveling, he would have been carrying water. If sprinkling or pouring were acceptable practices, there would be no reason for Philip and the eunuch to go down into the water. Thus, from this passage we learned that baptism is associated with water and that it is an immersion. Also notice that by inspiration, the apostle Paul described baptism as a burial. Thus, we see that baptism portrays Christ's death, burial, and resurrection.
Baptism is to wash away or cleanse one of sin and, thus, frees one from the bondage of sin.
Before one is baptized, he or she must be taught. Remember in the Great Commission, Jesus said to teach and then to baptize. Saul was first taught by Ananias and the eunuch was first taught by Philip. In each example as a result of teaching, they believed that Jesus is the Christ. Thus, belief must also precede baptism.