Integrated eye-tracking enables foveated rendering, a technique that creates sharper, more realistic images by blurring the scenery around the user's gaze. This means there's less power being spent on things that users aren't actually looking at.

#HTCVIVEPROEYE integrates eye tracking natively & will offer greater accessibility, gaze-oriented menu navigation & removing the need for controllers. #HTCVIVE pic.twitter.com/QTAWOgElIW — HTC VIVE (@htcvive) January 7, 2019

Integrated eye-tracking removes the need for controllers in certain scenarios, such as menu navigation, according to HTC.

Last year at CES, HTC revealed the Vive Pro and the Wireless Adapter that turned that headset into a standalone device.