This is clearly explained on the wiki page on the Master (emphasis mine):
The ROS Master provides naming and registration services to the rest of the nodes in the ROS system. It tracks publishers and subscribers to topics as well as services. The role of the Master is to enable individual ROS nodes to locate one another. Once these nodes have located each other they communicate with each other peer-to-peer.
The Master also provides the Parameter Server.
The 'phonebook' aspect of the master can be compared to how a DNS server functions.
Note that ROS2 does not depend on a master to locate nodes, it is fully peer-to-peer.