Toast matters because software becomes usable when empty, loading, error, success, mobile, and accessible states are intentionally designed. It gives builders a precise word for the thing they are changing, debugging, reviewing, or shipping.
ELI5
Think of Toast as a visible piece or state on the screen. It gives one small job a clear name so the whole app is easier to understand.
In practice
Use it when reviewing whether a screen tells users what happened, what matters, and what they can do next. In practice, name the owner, input, output, failure mode, and proof before relying on Toast.