In our religious culture, tithing is synonymous with giving. It’s not supposed to be that way, of course—tithing is an Old Testament practice, and we are living under the New Covenant—but when you talk to someone about tithing, they’ll generally equate it with what they give to their local church.
In reality, tithing was a set amount (a strict 10% across the board) and mandated. It wasn’t optional; without the tithes given to the Levites, the work of the Temple and synagogues would cease.
But there was another tithe (actually, according to scholars, there were two more). Contrary to the one given to the Levites, however, this one was consumed by the family themselves as part of their annual pilgrimage to the Temple.
According to Deuteronomy 14:22-27, every year, jews were obligated to travel to the Temple. That would naturally incur expenses: food, lodging, donkeys, and the necessary stops at every single tourist trap along the way.
As it turns out, God built that into the Law. These verses tell the family to tithe out of their yearly produce—specifically, that which comes from the field—and then eat it in the presence of God, which is in Jerusalem.
In a way, God is telling them to pre-fund their yearly pilgrimage, so that when the time comes for them to go, the money is already set aside. It’s as if God mandated that we head to Kansas City every Spring for a week, and He told us to set aside 10% every single week for that specific trip.
And, just like with the other tithes, it wasn’t optional—it was mandatory.
Why does God demand Israel to do this? The answers could be as varied as you want, but presumably, it all goes back to making this journey a priority. If you plan to go to Jerusalem every year, but never set aside the money, how likely is it you’ll go?
God wanted to remove any obstacles that were in the way of His people serving Him, which includes financial obstacles. It worked out better for everyone, anyway; God received the worship of His people, and the family got to spend a week in the presence of their God. What could be better?