A backup and comparison. And I can imagine Google uses it partially to determine value of a website in ratings. I never researched this to confirm it or not.

I'm not sure it's that expensive if you consider the value it can add. I would say you should be actively using it, and having a large amount of experimenting going on while using it. Also, you could discontinue using it after you've done testing that shows the positive results you want - and while not experimenting, then not using it. You won't have long-term or cohort results then necessarily, but I imagine they are targeting high revenue or high revenue potential businesses.