Because the author was nice enough to provide a Vagrantfile that can easily spin up a box that gets provisioned with all the required software installed. Sure you could probably run it on your host OS, but why would you even want to bother with that?
definitely you can use your local OS to run it. if you have a full LAMMP stack ready, just setup a new vhost (or whatever your typical process is.. .and off you go).
(as a matter of fact, that's how it would be deployed in production).
however vagrant is perfect for local, since you need mongo and mysql and various php extensions, etc. etc. all of that comes pre-packaged w/ vagrant... so if you don't feel like littering your local with a bunch of libs that you or many not use in the future, use vagrant.
