Activation matters because shipping code is not the same as proving activation, conversion, retention, or revenue. It gives builders a precise word for the thing they are changing, debugging, reviewing, or shipping.
ELI5
Think of Activation as a scoreboard for what people do in the app. It gives one small job a clear name so the whole app is easier to understand.
In practice
Use it after a feature ships to verify real behavior instead of relying on screenshots or vibes. In practice, name the owner, input, output, failure mode, and proof before relying on Activation.