One way to set up a mystery is to give the heroes something that the players won't think is unusual... until you have someone in character point out how unusual it is within the setting. A map seems like a normal piece of information to most people, and in a low tech setting it might be the easiest way to show the route to a particular place. But once you have a global coordinate system and easy travel, it's much easier just to give a set of coordinates.
Think of other examples of mundane items that would be grossly inefficient, useless, or otherwise unusual in other settings. Physical cash in a setting where transactons are all electronic. An elaborate key, in a setting where locks are just simple latches. A knife, in a land of ten thousand spoons.
Commentary by Keybounce (who has not seen the movie)
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Commentary by memnarch (who has not seen the movie)
Wait, what? C-3PO's got a red arm? Huh, I somehow missed that detail in the previous comics. The sunset colors did an excellent job of camouflaging that change. I wonder if that ever gets a mention in the movies or if it's a "read the tie-in comics to find out!" sort of thing. Translator droid doesn't seem like the sort of occupation that would carry amputation risks. And it has to be an important detail of some sort too, otherwise a new golden arm would be just as good for making a repair. If the audience can't see that a repair was done, then it never happened, right?
BB-8/Corey seems rather unconcerned about that doom mark now. If the location is discovered or understood, that would trigger the curse right? Maybe now he thinks it's possibly just a curse on the physical parts of the map, so that's all that needs to be held onto. Or possibly that as a droid, it's much easier to be repaired/fixed than a squishy organic being like Lor San. Also possible is that Corey simply forgot about that detail. Again. If there's enough exciting stuff between then and now, I could easily see that being forgotten; in-person games like this don't have an easy-to-search record of what happened.
Also, that is an excellent question now that it's pointed out. A map is only usable as decor if there are no points of reference on it to start from. Thinking about this whole thing more, it kind of feels like this is an Irregular plot point. Having (what I presume is) a map is better for showing not telling a location, and Star Wars is very big on the showing side. So this would more likely be a discussion about how dangerous Ahch-To is instead of being about where Ahch-To is. Or it's just how coordinates for important stuff are shared. Or potentially even a safe route for traveling through an exceptionally dangerous part of space.
BB-8: Now that we have the resources of the Resistance available, can we take another look at this map?
C-3PO: You have a map?
BB-8: Showing the way to the lost world of Ahch-To. Supposedly Luke Amidala made it.
C-3PO: Why would he make a map, instead of just giving Galactic coordinates?
BB-8: Is that a good question?
GM: Yes. Very good.
BB-8: That’s a very good question.
C-3PO: This map doesn’t match any known region of the Galaxy.
BB-8: Yeah, we knew that already.
C-3PO: You seem big on mysteries but short on answers.
GM: Like the mecha campaign.
BB-8: Hey, that was mostly her fault.
C-3PO: So it looks like you’re learning.