Hello David
While it might seem redundant, the two commands actually serve different purposes in Cisco IOS.
The command logging monitor level is used to set the severity level of messages that will be logged to the terminal lines (VTY). The level parameter specifies the types of messages to be logged. For instance, if you set the level as ‘debugging’, it will include lower severity level messages such as informational, notifications, warnings, errors, critical, alerts, and emergencies.
On the other hand, the terminal monitor command is used to enable or disable the display of logging messages to your current VTY session. By default, logging to the terminal lines is disabled. So, even if you’ve set a certain severity level with the ‘logging monitor’ command, even though those messages will be logged, you won’t see the messages on your CLI until you enable them with ‘terminal monitor’.
I hope this has been helpful!
Laz