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Maybe I didn't read carefully enough, but no one suggested that the Grail could be found on large maps. And that's when Conflux becomes really useful. Knowing all the spells – what could be better? Although, I don't particularly like Conflux myself.
Tower, in this case, allows you to reveal the entire map – but if you've already found the Grail, I think you'll only be interested in exploring the rest of the map out of pure curiosity.
For the Necropolis, the Grail is useless. It's too specialized.