B.W. Webb has been a bit of a journeyman in the NFL. He was a fourth round pick of the Dallas Cowboys in 2013 and the Bengals are now his 8th NFL team. He didn’t even play a game in 2017, after being cut by the Bears in September.
William Jackson and Dre Kirkpatrick aren’t going anywhere, so Webb was presumably brought in to replace Darqueze Dennard in the slot, though Dennard was also brought back. While Webb does have experience playing this position, 2018 was his best season as a pro, and he played mostly outside corner.