What is a startup script?
A startup script runs during a system’s initial boot up; it is applied to a system using a group policy. Startup scripts run under the context of the local computer’s SYSTEM account.
Why use a Startup Script for ConfigMgr?
To check configuration settings and the state of services that the ConfigMgr client agent depends on for successful operation as well as install the client agent if it is not install or functioning properly.
Why use a Startup Script instead one of the built-in methods to install the client agent?
A startup script avoids most common DNS issues, firewall issues, and other connectivity issues that are common when a central system attempts to touch all of the clients. It also adds the health/configuration checks mentioned above to check for (and correct some) dependencies as well as report on systems still having issues.