HDR support is now available on Google Play Movies and will be offered for a portion of films. The movies offering HDR are available at the same price as regular 4K movies, which were added to the platform in December 2016. The inclusion of HDR separates Play Movies from some other popular platforms.
By typing "HDR" in the search field on the site, you can find all of the supported titles, of which there are quite a few.
HDR can enhance a movie's contrast and color reproduction, giving it a noticeably better look. You need to have an HDR-ready display for the feature to matter.