The Second Tithe (Old Covenant)
God ordained many festivals, three of which He required all the males of Israel to attend each year in Jerusalem, in order to prepare His people for the coming of His Son, the Christ. Because the land of Israel was relatively large considering the means of transport in those days, Yahweh wished to make sure the people would have enough saved up in order to keep His commandment. Because of this He commanded a second tithe to be taken year by year.
22You shall surely tithe all the increase of your seed, that which comes out of the field year by year. 23You shall eat before Yahweh your God, in the place which he chooses, to cause his name to dwell there, the tithe of your grain, of your new wine, and of your oil, and the firstborn of your herd and of your flock; that you may learn to fear Yahweh your God always. Deuteronomy 14:22-23
As mentioned, knowing that the journey to Jerusalem would be very long for some of Israel, He allowed them to convert their tithes into money, in order to make their long journey less cumbersome.
24If the way is too long for you, so that you are not able to carry it, because the place is too far from you, which Yahweh your God shall choose, to set his name there, when Yahweh your God shall bless you; 25then you shall turn it into money, and bind up the money in your hand, and shall go to the place which Yahweh your God shall choose. 26You shall trade the money for whatever your soul desires, for cattle, or for sheep, or for wine, or for strong drink, or for whatever your soul asks of you; and you shall eat there before Yahweh your God, and you shall rejoice, you and your household. Deuteronomy 14:24-26
We see that God indeed planned for there to be a changing of goods into money at His chosen place. It was men who profaned this commandment by looking to make a profit thereby (Matthew 21:12-13).
This second tithe was important, because it allowed God's People to commune with Him at the appointed times. Whereas the first tithe was solely the property of Yahweh for the Levites, the second tithe was primarily for the offerer himself, although it was still to be shared with the Levite. Under the Old Covenant, Yahweh's Spirit did not settle permanently upon people as it blessedly does under the New Covenant of Christ. Thus, His people had to make the best time out of the three chances they had each year, at the place where the Name of Yahweh dwelled.
That which was either brought to, or bought at, the Place of Yahweh's Name, was to be eaten with joy before Yahweh Himself! It was a time of communion with their Father, the King of Israel. It was a time of fellowship with their families and their friends. The Apostle Paul refers to this when speaking of the Festivals of Yahweh. Paul knew that to eat at the altar is to commune with the one to whom is being sacrificed:
18Consider Israel according to the flesh. Don’t those who eat the sacrifices participate in the altar? 19What am I saying then? That a thing sacrificed to idols is anything, or that an idol is anything? 20But I say that the things which the Gentiles sacrifice, they sacrifice to demons, and not to God, and I don’t desire that you would have fellowship with demons. 21You can’t both drink the cup of the Lord and the cup of demons. You can’t both partake of the table of the Lord, and of the table of demons. 1 Corinthians 10:18-21
It was therefore very important that the children of Israel remember this commandment, to keep year by year a tithe (tenth) in order to enjoy themselves the following year at each of the three festivals. This was not to keep the House of Yahweh full for the priests benefit, but to bring the people themselves into communion with Yahweh. There was no other time of such intimate fellowship with God than at these three times, and thus no male was to appear "empty-handed" (Deuteronomy 16:16).
We also see that God again remembers His servants, the Levites, by making sure they receive a portion of the second tithe as they have need, in addition to receiving all of the first.