The following happens in many software projects -
At start, it seems you only need one environment for your web application, well, at most two:
One development environment (AKA your PC) and one server.

But as time pass, you find you need additional environments:
The clients might want their own testing environment, sometimes you need to have a pre-production environment or a staging environment, so business managers can approve the ongoing content as well as look & feel.

Do you really need these environments? What are these environment good for?

Here is a short description of some of the more popular environments and their purpose.

Cross browser, cross operating system testing is a costly and tedious task. Most often we test our web application on the Internet Explorer and Fire Fox installed on our machine and hope for the best. In big projects with rigid compliance requirements we test the major operating systems (Apple, Windows and sometimes a popular Linux distribution) with the major Browsers. The underline assumption is that this covers 99.x% of the population and we are happy with that. But what if we could, without additional cost, see how our web site looks like in many operating systems and multiple browsers?