Bridge has a few personalities, neatly explained in this tutorial: http://forum.mikrotik.com/t/routeros-bridge-mysteries-explained/147832/1
One of personalities is interface allowing CPU to communicate with L2 network joined together by bridge (the switch-like personality). If CPU is to communicate on L3 with devices members of said L2 network, then this interface needs IP address. Of course, if device is used as switch, then bridge interface is not needed and thus doesn’t need IP address. But it gets handy if one wants in-band management of such switch.