Yeah - why should a user switch over from Pathogen or Vundle? I'm not trying to be a jerk -- just suggesting that that explanation is probably worth giving.
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.
The biggest reason over Pathogen is automatic installation of your dependent plugins. Biggest reasons over Vundle is parallel downloading, dynamic plugin loading, and static plugins. (all of the reasons for me, YMMV)
I've found even that I can install vim-plug with only a few lines of vimscript so that I don't even need to store a .vim directory in my dotfiles repository. Everything can just be installed and run the first time I start vim on a machine, and I can easly :PlugUpdate to get updates to any of my installed plugins.
FWIW, this is also quite possible with Vundle. I keep my plugin management in a separate file which also contains the Vundle bootstrap logic for new systems. This stanza sets up Vundle if it isn't already installed...:
It doesn't do those two things, but I don't think that's a compelling argument for why it doesn't manage plugins. I certainly use it to manage my plugins.
[1] https://github.com/tpope/vim-pathogen
[2] https://github.com/gmarik/Vundle.vim