Key Event is a GA4 event marked as especially important to the business, such as a lead, purchase, or signup.
It matters because analytics turns marketing activity into decisions. Without clear measurement language, teams may misread performance, overvalue the wrong channels, or miss the actions that actually drive revenue.
Example
Example: A business might mark generate_lead, phone_call_click, and appointment_booked as key events in GA4 because those actions represent meaningful progress toward revenue.