Simply having customers incurs costs in support. The more customers you have the more different hardware and OS patch and ancillary software configurations your app is running on, the more uses your app it put to, the more problematic edge cases in the UI and functionality and bugs will be encountered, the more feature requests you will get, the higher the chance that old versions of your app will be in use for longer.

Eric's exactly right, you're providing a service and service costs scale up with users. Not all of them linearly, but some for sure.