Large shops that already run on AWS and where iOS/mac is a small part so they'd rather dish out a few $100k for someone else to manage and have it homogeneous through their other systems.
And it's a standard service that any of their developers can access as needed without needing to either setup their own Macs as a service in a lab or distributing standardized Macs for testing/builds/etc.
For a small shop it probably makes more sense to just buy the Macs, especially if they do a lot of iOS/OSX development.
For a small shop it probably makes more sense to just buy the Macs, especially if they do a lot of iOS/OSX development.