It was fairly agnostic to the actual contents of the channel as long as it fit within the assigned bandwidth. For example, they started using broadband digital modems to carry PCM audio by the 1970s, in parallel to the TV and FM voice. The circuit and packet-switched computer networks AT&T later provided were first carried this way too, as I understand it.
In practice, NBC/ABC/CBS were the biggest clients and used it to relay live network programming, etc. until satellites started displacing that in the 1970s. But it got used for all sorts of esoteric purposes too. Anything you needed bandwidth for, if you could afford it.