Intersection Observer matters because lazy loading, reveal animations, and infinite lists should observe visibility instead of polling scroll constantly. It gives builders a precise word for the thing they are changing, debugging, reviewing, or shipping.
ELI5
Think of Intersection Observer as a named piece of the app screen or browser toolbox. It gives one small job a clear name so the whole app is easier to understand.
In practice
Use it when checking how users interact with the page or how browser code behaves. In practice, name the owner, input, output, failure mode, and proof before relying on Intersection Observer.