I'll start by saying that the mapping action isn't the most important or urgent thing in the world, and certainly if we're going to replace any Waystone action for a personal project that one should be the one to go. That said, I believe that it does have value.
The purpose of the mapping is mostly to determine how many weak points in the Old World's network exist so that we can respond to any threats to them. Mapping the Empire, for example, confirmed that Marienburg's nexuses are kind of important to the survival of the Empire. We believe Estalia contains one of only two paths for magic to flow from the Old World to Ulthuan, and if that path is blocked the entire Old World is done for, so mapping it will allow us to know which Estalian cities we can continue not caring about and which Estalian cities are so important that the proper reaction to receiving news that they're in danger is sending an army right away.
Tilea is genuinely less important in that sense, because it probably doesn't connect to the Empire's network (though it might be conneted through Bretonnia). My main concern there is the existence of Skavenblight, which probably contains a nexus and I'll really rather know if any leylines are still connected to it. If, for example, we find that the chain of leylines that starts at the Border Princes and continues into Tilea terminates in Skavenblight, then that means that improving the Waystone network in those areas will also send additional magic to the Skaven. That's the other general thing mapping can give us: it can allow us to discover lost or corrupted nexuses, which is something we'll eventually want to deal it. For example, mapping the Empire would have allowed to discover the Mordheim nexus, if not for the fact that we happened to visit the Gross Selon nexus during the Ostermark mini-arc and so knew about it beforehand.
Regarding the question of what we'll do with what we discover, I think it's true that the project itself won't immediately act on that information, though it could give us some new options e.g. new nexuses we might want to investigate. But if we give the Emperor a list of cities that would doom the Old World if they fell, and then an Everchosen candidate sacked Bilbali or w/e, then he would know to March of the Damned Soldier some poor sods to deal it with right away because it's an existential threat, and that seems like a pretty big win.
