The ability to run multiple instances at once with different configurations for each instance which makes it easier than having one big server running all your apps in production mode (which can be very expensive).
It's not as easy or intuitive when you're new but after some time using VF I realized how powerful this tool really was compared other similar tools out there like Appirio etc.. We have been able solve many issues related security compliance by just installing few modules from within virtual forge itself without needing any third party software/tools!