From DoomWiki.org

Puzzles are similar to the more intricate traps, but don't offer a direct threat to the player and usually provide a reward when the puzzle is figured out. As a simple example, you might have one switch temporarily expose another switch, perhaps out of direct view of the first switch but within running distance. The player, in that case, must recognize the sound of a wall lowering or a door opening and locate the second switch before it is hidden again. Finding the second switch in time might grant the player access to a weapon or powerup. When used in moderation, puzzles can add a great deal to the enjoyment of a level.