This table shows every device that has ever been discovered in all snapshots (their Hostname, IP address, Management protocol used for discovery, Serial Number, Username used for discovery and Last discovery time filed).
All of this is important for keeping track of all discovered devices. Also, information about these devices are used in every snapshot and devices from this list are marked in the Connectivity Report as previouslyDiscovered devices.
Sometimes it can be helpful for troubleshooting purposes to delete some of the already discovered devices. All discovered devices can be found in this table and any or all of them can be deleted by selection and delete button on the top.