(redundant bit) As someone else has pointed out, this is incorrect - the Squeezebox players decode FLAC, MP3 etc. directly from the stream.
(non-redundant bit) And the Squeezebox Server is a very useful central controller for setting up plugins, playlists etc.
I guess the Sonos players can also synch to each other tightly so that playing the same thing in multiple rooms works nicely? The physical Squeezebox players do, but the version you might run on a PC (Squeezeslave) doesn't as far as I know.
And if you're paying in GBP, you'll probably be interested in the BBC iPlayer plugin which is very convenient. No neat Spotify integration yet but it's on the cards.