God gave his people three festivals to celebrate each year: Feast of Unleavened Bread, Feast of Harvest and Feast of Ingathering. Each festival was to be celebrated at the place where the tabernacle or temple was located. For most of Isreal's history, that place was Jerusalem. Each time, they were to bring an offering. "No one is to appear before me empty-handed," God told them.
I wonder why. God certainly didn't need any of their gifts or offerings. Later in the Bible he says that he desires mercy, not sacrifice, yet each time the people of God came to the place where he dwelt, they had to bring something with them. Yes, some of what they brought was to help sustain the priests and Levites serving at the tabernacle/temple, but I think God had another purpose in mind as well. Could it be that he was teaching his people to be a generous people? To hold loosely in their hands the things God gave them and to remember that all things--their crops, herds and children--were gifts and blessings from God? Certainly carrying out this instruction kept their focus on God.
What about today? Are we to bring something to God each time we come before him?