Whenever you save a TypeScript file in your SPFx project, the build chain creates a new build and refreshes the local workbench automatically.
There are some situations you like to have this support for other asset or frameworks too. In most cases with the PnP/SPFx generator, we need to force a build when a Handlebar or VueJS file get saved.
So the goal is to hijack the build process and add some custom file watch based on the requirement of the framework or file. Why file? Imagine you have an SVG file in your solution. The expected outcome is that you see the changes immediately in your browser.
The key word to this is watch. It requires a custom file extension watch that than trigger the build.