This concept has come up in our reading before, but it bears repeating. Those who serve God, who are called to teach and lead His people, are to receive their daily bread from those they serve. There are some really practical reasons for this. It meant they would not get embroiled in squabbles over land or herds or wells. It allowed their attention to be undivided. Serving bi-vocationally often results in being torn between the role that is putting food on the table, and the one to which God has called you.
But it also helps the people being served better understand their role in giving. God does not need our money. The purpose for our giving is not because God has needs that our money can meet. God owns it all and gives it to whom He chooses. It's not even for the purpose of taking care of those who serve. God is more than able to provide for those who devote themselves to His service with or without your money.
The purpose of tithes and offerings is to remind us that it is all His. We give back a portion in thanksgiving for all that He has given. We are to give generously because God has been generous with us. We are to support financially those who support us spiritually, so that they can do so without being distracted by their physical needs, and joyfully as a testimony of how God has been faithful to us.