Whenever a Book of Mormon prophet hands down the plates, it is because the prophet in question is anticipating his own death (Mosiah 29:46). All preparations must be made to establish a wise successor and place the sacred records in the hands of a worthy recipient. This is what Mosiah is doing in this chapter; he is preparing for his own death by defining a successor and a protector of the plates. The most likely candidates for both would have been his sons, but they were not interesting in the throne of Zarahemla. It is interesting that Mosiah doesn't confer the plates on any of his righteous sons. Instead, he confers them upon Alma the younger. This may have been directed by the Lord or this may have been because his sons had already gone on their missionary endeavors amongst the Lamanites (Mosiah 29:3). Although his sons were worthy of the responsibility, Mosiah instead chooses Alma the younger, who had no plans to proselyte amongst the Lamanites and would therefore be in a better position to protect the records. He also became the first chief judge and high priest over the church (Mosiah 29:42).