The feature allows WebXR applications to retrieve data about planes (flat surfaces) present in the user’s environment. This enables AR applications to create a more immersive experiences through the obtained information.
Without this feature, developers would have to resort to running their own computer vision algorithms on data from getUserMedia() in order to detect planes present in the users' environment. Such solutions have thus far been unable to match the quality and accuracy of native AR capabilities or support world scale.
Docs: https://github.com/immersive-web/real-world-geometry/blob/master/plane-detection-explainer.md
Explainers: https://github.com/immersive-web/real-world-geometry/blob/master/plane-detection-explainer.md