In the GCS and GCE Challenge Lab the startup script installs the stackdriver agent. Is this required? I created a new instance without any startup script and I am able to view stackdriver logs in the GCP console for this instance.
The stackdriver agent, soon to be called Cloud Logging Agent is not required for informational logs about the VM instance.
Installing the Logging agent allows you collect logs from third party applications, think nginx or apache web server or database instance logs, which are not collected by default. Allows you to get the full picture of both the VM and the applications running on the VM.
Here a link of how to configure various third party apps to send logs: