Best answer by Jeff S
Welcome to the community. The ZP120s, or Connect:Amps as they're known these days, are generally used to power third party speakers. The ZP90, or Connect, is used as an input and output device, most often to integrate an existing receiver or powered speaker system.
The Connect:Amp can power speakers to be used as rear speakers in a surround setup, but only if the front channels are handled by a Sonos home theater product. The Playbar, Playbase, or Beam can be used to accept audio from your TV as front channel speakers in a surround setup. The Connect:Amp can power the rear speakers in this surround setup, but needs to be wired into the network with an ethernet cable as they do not have 5 ghz antennas needed for wireless surround. You can read more about using the Connnect:Amp in a surround setup here.
Outside of a home theater setup, each Connect:Amp you have can act as its own "room" that can play music independently, or in sync with another Sonos unit if you wish. The Connect works in a similar way, creating a Sonos room that can play music to speakers connected to it, or playing audio from an input device, such as a CD player or receiver.