Why do we call God, Father God?
We find calling God, just God, very cold and impersonal. Yes He is God, but He is also very much our Father, so we relate to Him that way.
We are adopted sons and daughters with free unlimited access to our loving Father.
Jesus called Father God, Father (Luke 23v34) If you reading the new testament, you can tell the level of deepness and intimacy Jesus has with Father God.
Calling God Father, had a massive impact on my prayer times because I then started to relate to Father being very close and present and not distant like you would maybe imagine God to be.