The Proclamation of 1763 stated that American colonists could not settle beyond the Appalachian Mountains. The main reason for this was

I belive the answer is: to maintain peace with the native Americans.

The proclamation was made by the British empire after they made an acquiestion of The french's empire territory following the damage that the french received during French and Indian War/Seven Years' War.

Even with the border agreement, The great Britain still able to obtaina hefty sum of land.