Some of the features:
* Quickly preview or jump to figures/references/equations/etc. (even if the PDF doesn't have links)
* Search paper names in google scholar by middle clicking on their name
* Searchable table of contents
* Searchable highlights/bookmarks
* Browser-like history navigation
* Mark locations for quick navigation (Vim style)
* Synctex support
Video demo of some features:
https://www.youtube.com/watch?v=yTmCI0Xp5vI