'Futile' in the Bible

“Oh, that there were even one among you [whose duty it is to minister to Me] who would shut the gates, so that you would not kindle fire on My altar uselessly [with an empty, worthless pretense]! I am not pleased with you,” says the Lord of hosts, “nor will I accept an offering from your hand.

You said, "It is futile to serve God,' and, "What did we get out of it when we carried out his requirements and went about like mourners in the presence of the LORD of the Heavenly Armies?'