Deuteronomy 14:25 Then shalt thou turn it into money, and bind up the money in thine hand, and shalt go unto the place which the Lord thy God shall choose:
When God was teaching His people about their offerings He told them to take the offering of the increase of their fields, etc. They were to take it to the Tabernacle and later the Temple. If they lived too far to make it convenient then they could sell their increase, change it to money, then take it to the temple, and purchase their sacrifice.
Money is a convenient way for us to honor God with all that He has given us. We do not harvest fields today much at our church but we do reap the benefits of our jobs. We get money from our work and honor God by giving Him the first part of it. He wants it to be convenient but they understood that they were to give.