GA4 matters because GA4 setup affects how events, conversions, funnels, and audiences are measured. It gives builders a precise word for the thing they are changing, debugging, reviewing, or shipping.
ELI5
Think of GA4 as a proof or operations tool for the live product. It gives one small job a clear name so the whole app is easier to understand.
In practice
Use it when launch, analytics, revenue, reliability, or production proof matters. In practice, name the owner, input, output, failure mode, and proof before relying on GA4.