OK, many receivers now offer the facility of connecting several HDMI sources to them, and being able to output the one you want to watch via a single connection to the TV. That's convenient, as it allows the picture and sound to be switched together - ie you select satellite, and the picture from your Sky box appears on the screen and the sound from it comes from the speakers. Switch to DVD and you get DVD picture on screen and DVD sound from speakers.
However, not all receivers with HDMI inputs and outputs can process the sound signals available on the connection - they merely switch the picture, and you need other connections for the sound. And not all source components give the best sound on HDMI - a Sky box, for example, will only give stereo via HDMI, and needs an optical digital connection you your receiver to deliver surround sound.