It's not that testing with real devices is the gold standard, though that's one way to look at it. The fact is, there is simply no way to emulate the behavior of a mobile app in the field unless the device is literally in the field. System overhead, memory usage, CPU speed — a host of variables impact app functionality in ways that just can't be reproduced in a lab. To get real test results, you have to deploy real devices.
But creating an internal test program using real devices is a challenge. The devices have to be bought, carrier contracts have to be established, test scripts created, users trained, results compiled and analyzed — for each geographic market. And then the devices and contracts have to be dealt with after the testing is done, or maintained for ongoing monitoring.
This complexity is why so many enterprises use an outside test partner that offers an established infrastructure with hundreds or thousands of devices deployed over a broad geography. The test provider works with the client company to develop the necessary scripts, and then leases time to them on its network to run the tests. This is testing in the "public cloud," which means that many clients utilize the same devices and infrastructure to conduct their tests. It's an ideal solution for most companies — there's no upfront capital expenditure and tests can be quickly executed on demand, on a budget-friendly pay-per-use basis.