What is the primary purpose of God uniting Himself to us as members of the Body of Christ?
The primary purpose is fruitfulness: God unites Himself to believers as members of the Body of Christ so they may be fruitful and bring forth spiritual children. Using the marriage analogy, Paul teaches believers are joined to the risen Christ to produce fruit unto God.
In Romans 7:4, Scripture says believers are become dead to the law by the Body of Christ and are married to another, even Him who is raised from the dead, that they should bring forth fruit unto God. Les explains that just as the primary purpose of God bringing man and woman together in marriage is fruit—children—so the primary purpose of God uniting Himself to us as members of the Body of Christ and claiming us as His own is to be fruitful and to bring forth spiritual children.
Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God.