https://youtu.be/pckxBN0PJD8
Deployment service is rarely used but it is an integral part of deployment process, when you deploy an app it gets executed. You can also customize the deployment service to add calls and scripts that perform specific functions when a process application is deployed on a server in another environment.
If you disconnect the line from start to end for the default deployment service which is automatically created for every process app the deployment will fail. There can only be one deployment service for a process application.