Vows and tithes

Leviticus 27:1-25
Anyone makes a special vow to dedicate persons to the Lord by giving equivalent values.
If a man who dedicates his animal, house, part of his family land, redeems it, he must add a fifth to its value. Priest will judge its quality good or bad and value it when a man dedicates it.
If he does not redeem the field, which he dedicated to the Lord, of it he has sold it it to someone else, it can never be redeemed. When the field is released in the Jubilee, it will become holy like a field devoted to the Lord, it will become the property of the priests.
If dedicates to the Lord a field he has bought, which is not part of his family land, the priest will determine its value up to the year of Jubilee, and the man must pay it value on that day as something holy to the Lord.
Why?
Firstborn sons, the firstlings of flocks and herds, and firstfruits of the field are God's by right (he accepts part for the whole)
How?
All I have are belongs to the Lord, I must pay part of it to the Lord like tithe to express my faith to God.