Seen it regularly when visiting towers in Austria. It doesn't need to have anything to do with a visual approach, most of the time, a/c were vectored for instrument approach.
At airports with mountainous terrain around, you have a lot of minimum vectoring altitude sectors. If you have an a/c at FL120 and want to descend it to 4000, but have sectors of MVA of FL100, FL90, FL80, 060A each spanning just a few miles along its route, it can increase your workload. After establishing that a/c is visual with terrain, you issue a "descend visually to 4000ft".
I won't give you any reference to regulations as we dont have to use it at my unit.