In Psalm 147:15–18 the focus is not the people of Israel anymore; now the psalm extols the greatness of the Lord. This verse says that God’s commands and God’s Word go quickly toward the earth. The image is that a herald delivers the message from the king as quickly as possible.
In this verse we see the connection with Isaiah 55:11, where it says that God’s Word shall not return to him empty. When the Word goes out (especially in the preaching) it always has results, be it positive (a deepening of the faith) or be it negative (a rejection of God’s love). Thus, the importance of God’s Word and attending church worship services is a given.
15 He sends out his command to the earth; his word runs swiftly.