Part of the problem in explaining it is that 'streamer' covers such a broad range of possible capabilities. At its most basic you've got something that has a network connection, an audio output (analog or digital) and nothing else - you control it from an app, a browser or some other control device like the Squeezebox Duet. At the other you have something like the Squeezebox Boom or Radio that also has a screen, buttons and knobs for the interface, and its own speakers. Or with inputs too, that you can stream to another endpoint on the network. Some can synchronise multiple endpoints so you can have the same music playing in different rooms.
I use LMS to play a mixture of locally stored content, Qobuz and internet radio sources to different sorts of players in different rooms, sometimes synchronised, or moving the current playlist from one room to another. Depending on the situation I control it with an app, a browser or the controls on the streamer (the Boom in the kitchen). Sometimes the 'streamer' is squeezelite running on a computer. I've even used it while on holiday, using a VPN to connect the laptop back to the network at home - useful when you find something's region locked, or only in your own music collection.