What’s a practical usecase for a bootstrap script since it only runs just the once at creation time, and not on reboots?
In my experience bootstrap scripts have been used when scaling EC2 instances. When a condition exists where more EC2 instances are scaled up the bootstrap script ensure each new EC2 instance receives the parameters (sometimes outside of AWS) indicated in the bootstrap script. This means each new EC2 instance will be autoconfigured.
A bootstrap script issues commands on the first runtime to install software and services. For example, the first time you spin up a web server, you have to log in as the root user, install any updates, and install and start the Apache web server. A bootstrap script automates the process.