To expound … this sort of echoes what @ramstrong said. I really like having maps as a tester because they quickly give me clues as to what might go wrong. I might say, for instance, okay–I’ll check all the dotted-line connections. Or I can check the room description immediately against the map, as anything I find there gives the programmer good value for their time. (It’s also the sort of thing I initially miss.)
My style is to go through a game and try to find ways to get puzzles almost-right, and having maps also saves brain power for trying to play Murphy’s Law. What would a person miss? What would a person almost figure out, but maybe give up if they needed to guess a verb?
Maps also help give a preview. For instance, if I get an email for a game I’m testing, I can have a look at the map so I know what I’m getting into later. The size, the number of secret rooms, the general structure. I’m not totally flying blind. Perhaps this ruins some of the emotional impact of the game, but that’s not my highest priority as a tester.
I like having and providing maps, because there are just so many games out there that we can’t approach all of them on their own from scratch. I’ve written a few, but not as many as I’d like. It’s fun, but it is work, like when I wrote for Luster, which was opaque. I do enjoy drawing and sharing maps too… but there’s a part of me who thinks it’s just REALLY neat to see others’ efforts and be glad I’m not the only one and be able to focus on other things, if I want.