Albert Barnes Commentary 2 Samuel 20:23

"Now Joab was over all the host of Israel; and Benaiah the son of Jehoiada was over the Cherethites and over the Pelethites;" — 2 Samuel 20:23 (ASV)

Now Joab ... - This is by no means a meaningless repetition. Joab had been dismissed to make room for Amasa and was now, as the result of his successful expedition against Sheba and the death of Amasa, reinstated in his command. Moreover, this was a fresh beginning of David’s reign, and therefore a statement of his chief officers is as appropriate as in 2 Samuel 8:16, when he had just established himself on the throne of Israel. (Compare to 1 Kings 4:2–6).