The most obvious benefit I believe is its ability to update plugins in parallel. A GIF [1] shows that it managed to update 40 plugins in mere 4 seconds. The other notable point is that it allows you to load plugins on demand, which helps reducing the startup time greatly [2] if you happened to use many number of plugins. It also provides some other nice features that pathogen or vundle do not have while still being easy to use.

[1]: https://raw.githubusercontent.com/junegunn/i/master/vim-plug...

[2]: http://junegunn.kr/images/vim-startup-time.png