Why does macau have their own flag?

Although Macau is under Chinese federal-control, it still only remains a territory. Therefore, as a Special Administrative Region (SAR), Macau is allocated to remain as autonomous territory where it has control over all government matters excluding military defense and international trade stances, similar to Hong Kong, another SAR. This level of autonomy also allows it to keep its own flag, similar to the States' Flags in the United States.