It is our duty, our responsibility, to present the offerings to God (chs. 28—29), whereas vows are voluntary. If we simply fulfill God’s requirements, we have not yet come up to the highest standard. In addition to carrying out our duty and responsibility to fulfill God’s requirements, God wants us to make a vow to Him, volunteering ourselves in a definite and particular way (Lev. 27; Num. 6). The higher our vow is, the more the Lord will use us to fulfill His economy.