Ethernet Transmission with one MAC and Multiple PHYs
Typically, a set of MII lines are connected from the MAC to a single PHY. The reason for multiple addresses for MDIO is for SOCs that contain multiple MAC modules and for switch chips. The MII from each MAC module connect to its PHY. However, to save pins on the SOC, there will be only one set of MDIO pins. The MACs will share these lines communicating with their PHYs, selecting each one by address.

In the case of Ethernet switch chips, where there would be many PHYs attached, there will be multiple MDIO sets to connect to subsets of the PHYs.