The tax of a half shekel (a weight, not a coin) at this time is similar to tithing in the Christian Church. You are offering an amount to assist the Church in upkeep and mission work. More importantly, the action of making an offering is a recognition of proper orientation in worship. You are giving something not because you seek to receive anything but that you have already received salvation from the Lord. The tax was also called "atonement money". The Lord had purchased the lives of His people by saving them from death, so now the people are living these God-given lives by following the Lord and helping their neighbours.