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[Hawkeye]

I have always bought the map for 2 coins, but I don't know what it does. Does anyone know what it is used for?

[matistjati]

if u press escape u can see the map and like which islands you have been on
i have a theory that it serves as a unlocking system since when i got to the second island i had an archer statue on the first island and dog on the second island
Every time i have restarted since (i suck) i have found a dog and archer statue which i hadn´t found before
just a theory

[Longtail]

Maps unlock 1 thing in the map you find it in. It unlocks Archery in first, Doggo and Different Horses in the second, Elk, Masonry etc. in the third map and so on. You have to jump between the maps and replay the previous ones to unlock everything. Once unlocked they will always respawn in the current map you play in.

[Phazonis]

As Everyone already said you need the map to unlocknew stuff like shrines and so on. Onl if you have the map you will unlock one of the unlocks on each Island. You can see how many each has by hitting escape and looking into the map there bottom right. if you dont get the map you can still move on to the next island but without new unlock.