Although Mormon knew that the records would eventually be lost, that event was in Helaman’s distant future. For Helaman, his task in his lifetime was to ensure their preservation, that thereby his people would be able to retain their knowledge of their covenants with God, and to, particularly, retain their understanding of the coming Messiah.
Verse 19 shows that even at this time the Lord had let Alma know that some form of their records would be preserved for the salvation of future populations just as they had served to convert the many thousands of the Lamanites during the mission of the sons of Mosiah.
Verse 20 ends the command to preserve the large plates and the brass plates. In the next verse, Alma turns to yet another record.