Per the disconnect-viserver documentation:
By default, Disconnect-VIServer closes only the last connection to the specified server.
If you have multiple connections to the same vCenter, not being able to specify which session to disconnect is problematic if you have multiple commands running independently. For example, if you have two connections to a vCenter, and the first connection is running a command or series of steps that finishes before the commands running against the second connection, if you disconnet-viserver, it will actually disconnect the session that is still actively running commands against the vCenter. Allowing disconnect-viserver to designate a specific session will avoid the need to track the sessions with additional code.