The biggest thing it adds on top of VirtualBox is a consistent workflow. If it helps, think of what Vagrant does technically as useless, and think of Vagrant only as a process discipline: to get a dev environment for any project you "vagrant up", to close it out "vagrant destroy", to access it "vagrant ssh", etc.

As soon as you get this consistent workflow, then you can teach it to your entire organization and the whole company knows that ANY project in the company can be worked on using this workflow without any dependencies on their own machine.

And if ops decides to suddenly change the dev environment from VirtualBox to say... AWS... nothing changes! I've had large companies (200+ developers) switch from VirtualBox to VMware to AWS for their dev environments without their developers ever really noticing.

That is pretty cool.

Of course, on top of this, you get a lot of nice things: you get provisioners, you get automatic network configuration, you get the entire plugin community, you don't have to worry about stability on your own because you tap the collective stability of thousands of users, etc.

But the real power is workflow.