When we "give to God", why do we do it through the local church? 

In the first half of the Bible (the Old Testament), God followers were commanded to bring tithes and offerings into the "storehouse". The resources were to be used for the ongoing ministry at the temple (Malachi 3:10).  Storehouse is an archaic word, but was a place to store food, equipment and valuables.  The tithes and offerings were used for the upkeep of the temple, ministry purposes and allowing priests to give full-time attention to God's Kingdom.

Today, we give to God through the local church for similar reasons. Among the many descriptions used for the church in the second half of the Bible (the New Testament), one of the most amazing ones is that the church is God's body (Romans 12:4-5, Ephesians 1:22-23, etc.)  That's right, the local church is God's hands and feet on this planet.  Collectively, the resources we give are to further advance God's Kingdom in our area. 

At Great Lakes Church this includes: mailers (inviting people to hear of God's grace), leasing Tinseltown Theater for a meeting spot, salaries for three people to give full-time attention to ministry purposes, operational expenses, local and world missions, children's ministry, and lots of tech equipment that's needed to continue adding video venue auditoriums (by the fall, we may actually be leasing every auditorium at Tinseltown)!