Tier One ISPs
They're for the global telco's. Tier One ISPs aren't in the habit of changing their core routers very often as you're looking at hundreds of millions and into the billions in network investment before its necessary. So the core routers need to be able to scale to support increases in the data transmitted.
At this level they're basically designing equipment to specifications given to them by the telcos.
10 Years ago it was rare that carriers had a 10 Gigabit backbone themselves (they were just starting to come online and it cost the carriers an insane amount of money to implement), now I work for a (admittedly massive) manufacturing company that uses one between main sites and there are many companies that do similar. 10 Gig is effectively becoming common.
When you consider that a transatlantic cable operates in the terabits per second and there's a lot of the you can start to get some sense of the amount of 1's and 0's that are transmitted. Just imagine how much traffic google uses with it's hundreds of thousands of servers pushing data out. You've got to remember the internet isn't just the www , phone calls, data communication between businesses etc That's why these devices are needed and that's why they need to be as fast as they do.