Data Layer is a structured object used by tag management systems to pass event and variable data to tags.
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: When a form is submitted, the site can push an event such as lead_form_submit into the data layer. GTM can then use that event to fire analytics and ad conversion tags.