When an aircraft is on a visual approach, you do not need to immediately clear for the visual if you want them to maintain the heading and altitude you previously assigned. The pilot must be visual before the controller can clear the aircraft for the visual approach, but once you clear them for the visual you give them the ability to turn. If you don’t clear them, even if they report the airport in sight, they can’t turn, so just keep that in mind as keeping them can prove to be helpful in certain situations.
1. 10.10.1 of the Infinite Flight ATC Manual, pulled from the original.