Why is it added, "and sitteth at the right hand of God"?

Because Christ is ascended into heaven for this end, that he might appear as head of his church, (a) by whom the Father governs all things. (b)

(a) Eph.1:20 Which he wrought in Christ, when he raised him from the dead, and set him at his own right hand in the heavenly places, Eph.1:21 Far above all principality, and power, and might, and dominion, and every name that is named, not only in this world, but also in that which is to come: Eph.1:23 Which is his body, the fulness of him that filleth all in all. Col.1:18 And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence. (b) Matt.28:18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. John 5:22 For the Father judgeth no man, but hath committed all judgment unto the Son.