God’s very identity as God obviously is itself sufficient reason to worship him (see the first commandment). But the placement of the instruction in this paragraph places the need for worship within the context of John having just seen what must soon take place,
that is, the content of the book of Revelation. The glorious work of the conquering Christ, sinners' rejection of it, the plagues on disobedience, the opposition of the dragon with his beasts and the prostitute, and yet the triumph of God’s program of redemption as it culminates in the new Jerusalem all conspire together to justify—nay, require—worshipful adoration of the God who advances his kingdom and enhances his glory through the ages. This angel, present as he was in the worship of Revelation 5:1–14, knows what is right for John to do!
9 but he said to me, “You must not do that! I am a fellow servant with you and your brothers the prophets, and with those who keep the words of this book. Worship God.”