Standardised, platform independent, easy to install and configure tools. If you've got a somewhat complicated dev environment, it's really really easy to tell new developers "look, just install virtualbox, install Vagrant, clone the repo, run vagrant up, and then vagrant ssh". A few minutes later they'll be sitting at an ssh prompt at a fully installed and configured dev server, which is pretty sweet.

Sure, you could do it all with slightly longer and more detailed instructions and more hand rolled scripts, but it would be slower and more fragile. Vagrant doesn't really solve any new unsolved problems, it's just a bit easier than the alternatives.

(Also, I'm not sure I understand your argument against vagrant. Part of the point of vagrant is, yes, to try and maintain the same deployment tools for dev and producton. Why do you think using vagrant makes that harder?)