'Useless' in the Bible

"I wish that one of you would close the temple doors, so that you no longer would light useless fires on my altar. I am not pleased with you," says the Lord who rules over all, "and I will no longer accept an offering from you.

You have said, 'It is useless to serve God. How have we been helped by keeping his requirements and going about like mourners before the Lord who rules over all?