The people want to make sure that they are going to follow the Word of the Lord in their worship. They are going to actually have God regulate how they worship, which is common sense. If God is the One who commands us to worship him, then we ought to worship in the way that God commands, right?
So, worship is not just making up your own ideas, doing your own thing, and being as creative as possible. In fact, when people have tried to be creative in worship, it usually did not go well with them. Nadab and Abihu are a case in point; they wanted to offer strange fire before the Lord. In other words, they wanted to offer incense that had not been prescribed by God. It might seem like a small thing, but of course, God actually killed them right there (Leviticus 10:1–3). And so God is not only the One who commands us to worship him, but God is the One who commands how he should be worshipped.1
Brian Borgman
2 Then arose Jeshua the son of Jozadak, with his fellow priests, and Zerubbabel the son of Shealtiel with his kinsmen, and they built the altar of the God of Israel, to offer burnt offerings on it, as it is written in the Law of Moses the man of God.