In addition to drawing readers in to the action of praising, the "must" here might also reflect a religious commandment. In other words, it seems that praising God is a requirement for good medieval Christians.
He first created for men's sons heaven as a roof (5-6)
Instead of delving into the intricacies of theology, Caedmon focuses on the scriptural basics: God made us a sky, hooray.
for men earth, Master almighty (9).
Again, instead of hymning about other things God has given to humans, such as religious law, guidelines for living a sinless life, etc., Caedmon sticks to the concrete: an earth to go under the sky.