I am not sure what is the difference between stackdriver logging and stackdriver logging agent.
The stackdriver Logging is a feature of Google Cloud Platform. It also works with AWS. What is available in the Console is Reports/Logs/filtering e.t.c. that you can view. Now if you want to use the same filtering/log viewing capability for the logs you write from your application running inside a vm or write the vm logs to the stack driver logging interface you will need to install the monitoring agent.
Stackdriver logging available from the console is a centralized Logging/Debugging/Monitoring/Error Reporting mechanism to view/filter/export. If the source of that log is a VM then the agent on the VM will help create these entries in Stackdriver console.
Hope this helps. Thank you