This feature is essentially mixins for vertex/fragment shaders. The first practical use case was offered by comp500. They have "a wand/tool/gun which creates a rift that compresses the entire world to bring the block you clicked on closer to you."

The effect is achieved with some vertex transformations that need to run after other vertex processing is complete. It's possible other effects would need to run at the start or during fragment shading, etc.

Users of this feature need a performant way to enable/disable them and may need some custom uniform state. The need for custom uniforms implies a practical limit on how many can be injected and/or active at the same time, but these effects aren't expected to be very numerous.