What is the use of trackingFeature property in aem cq_dialog?

This was added across AEM Core Components a couple of years back. The purpose of the change was to align the components with Coral UI tracking functionality. See the Coral UI 3 docs for more information.

Judging by the comments on a related GitHub issue, Adobe use this API to provide some analytics as part of the opt-in aggregated usage statistics

Quoting from the docs:

You can help improve Adobe Marketing Cloud by sending Adobe statistics about how you interact with AEM. This information does not contain any data about your company’s site visitors and will only be used to help Adobe deliver, support and improve your user experience.

You can opt into usage statistics collection by using either the Touch UI or the Web Console.

It's likely that the component you're looking at is a proxy component that extends one of the Core Components.