Hi,
Can anyone please tell me what the "Admin MAC Address" in the "General" tab of the "Bridge" interface window is used for (I know it means assigning a static mac address to the bridge interface but why?).
Thanks.

Because otherwise the bridge gets the address of the first active member port. But this is unpredictable so the mac address of the bridge can vary. Therefore there is administrative fixed mac address you can use if you want.