In the above example, the user just wants to signup. They don't care about receiving a welcome email or being subscribed to a mailing list or anything else, they just want to register an account. That's a pretty good use case for just `POST /signup`, that hits the user service and spits out an event that the user has signed up.
Starting a new workflow when that event is published makes sense in this case.
An example when a workflow is started explicitly could be something like doing a fire system test. You could:
1. shoot off a command that starts the FireSystemTestWorkflow
2. the workflow sends a bunch of commands to test sensors and sirens
3. those things publish events that they're functioning correctly
4. the workflow waits for all of these events to come back
5. the workflow publishes a FireSystemTestedSuccessfully event
The nice side of this is that the workflow can respond if a sensor or siren fails and does what's called a "compensating action", ie: compensates for the deviation of the successful path by performing a corrective action like sending a command to start the device or notify a technician.