We needed a consistent way to test our site against multiple browsers and devices. By having access to different devices sporting different versions of each OS, we can ensure our customers are going to be getting a satisfactory experience. Without this, we are guessing as to how or what experience our users are getting.
It's also helpful in researching issues that our customers report when we know what devices/OS/browser they're using. Using a browser emulator doesn't cut it and having a live device browser to interact with the site is invaluable.