The game doesn`t provide any reason to have such treaties, in fact, game mechanics actively discourages you to give up any provinces, since you already paid to core them. Also, any nation that has "your" cores more or less ends up on the "decimate ASAP" list, because you don`t have to pay admin cost for province that is already your core.
Then, obviously, the game has to provide insight on why and what would a country want to gain something from you if it lost. How is it supposed to defend the new gains? What potential consequences should there be? In what case should a country flat out refuse such trade? Imagine the obvious exploits like having Great Britain give up a province on island, so you could pre-emptively ship your army there, colonial nations giving up a province in their colonies, so a land power simply ships it`s super army into new world and goes for conquest spree, ignoring the need for navy entirely...
Current system, although quite rough, works pretty well. You can only take what you can actually conquer, and peace treaty doesn`t allow any kind of unfair exchange, because, there is no exchange at all, winner takes, loser gives.
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