That comment reminds me of Doom. It used a table called a Reject List. The reason for the reject table was so the game would know where not to process. So the map is scanned ahead of time (when making the WAD file), and if there are monsters trapped in the walls to be released later, they won't be processed. So the game needs to make fewer line-of-sight calculations. If the player cannot see certain monsters, the monsters shouldn't be using processing power to aim at you.