Mappers had to look listen to the description of the DM and try to draw a "good enough" map. Very easy to get things mixed up or one square off, and have to erase and redraw. "A doorway to the left" can be confusing when you're are heading south and it on the right of your map. Or maybe the DM means the left of the map?

Bear in mind, there was often treasure hidden in secret rooms, so knowing where the unexplored space was could be pretty important.

Once in a blue moon there was a player who got a thrill from that, but most folks hated the hassle.