What is the number one reason for going with Virtual Environments? Is cost always the determination reason?

I confirm that the number one reason for going on virtual environments is the cost of realizing the project. In my domain, it cost alot less to study the performance and to understand the impact of any modification on the project than on doing it on a highly cost prototype.

Moreover, Virtual environment allows to implement additinal innovative tools in an easy way for enhancing th performance of the project. 

I guess cost mostly but in my case some applications require less resources that it is a waste to use a whole server for.