There were at least three reasons why the British government issued the Proclamation of 1763. We cannot say which of the reasons was most important.
First, the British wanted to keep the American colonies tied to Britain. If the Americans expanded out into the continent of North America, they would become less and less dependent on Britain for trade. This would hurt the British economy so Britain wanted to prevent expansion.
Second, the British wanted good relations with the Indians. The British (and many colonial elites) profited from trading with the Indians and did not want to jeopardize that trade. Therefore, they wanted to prevent the colonists from encroaching on Indian lands.
Finally, the British wanted to save money. The French and Indian War had been expensive and the British government needed to economize. One way to do this was to keep colonists from settling in Indian lands. If the colonists did settle in Indian lands, they would want military protection from Indian attacks. This would be expensive and the British government preferred not to incur that expense.
All of these were purposes of the Proclamation of 1763.