An angelic communication is received by Alma in these verses. What was the purpose of an angel being sent to Alma? First, it was to bring him comfort. Alma took quite seriously the wickedness of the people of Ammonihah. It weighed him down to the point that he felt "an anguish of soul." (see vs. 14) The angel commands Alma to be of good cheer, stating that Alma had been "faithful in keeping the commandments of God from the time which thou receivedst thy first message from him." Then the angel adds an extra bit of encouragement by saying "Behold, I am he that delivered it unto you." (vs. 15)
The second reason the angel was sent to Alma was to command him to return to Ammonihah to declare repentance to the people. There was a very specific reason though that was outside of the scope of just the people of Ammonihah that Alma was being sent to Ammonihah. In verse 17, the angel explains, "For behold, they do study at this time that they may destroy the liberty of thy people." The undermining of the Nephite nation, and a people that were bound by covenants to the Lord, is at the very heart of the motives of the people of Ammonihah. Suddenly, it's not an issue of letting wicked people just be and exist in their wickedness (which was hard enough for Alma), but now it is a issue of national security, and more importantly, the means by which the judgments of God may be rightly executed on behalf of the liberty and protection of those that do believe and follow the commandments of God.
Because He is a merciful and infinitely just Being, God will not execute judgments upon any of his children, without first sending messengers to warn them of the pending destruction which awaits them.
Returning to verse 15, and the angel's declaration: "Thou has great cause to rejoice." The implications of this statement are deep. The reason for the angel's reassurance is because of Alma's extended period of perfected faithfulness to the commandments of God. Prolonged obedience brings great stores of personal peace and integrity. That an angel sent from the presence of God gave Alma this reassurance gives me greater confidence (amidst humility) that when we are obedient to the commands of God, we will always be right.