You are missing the ability to send digital audio to the receiver. The number and types of formats has exploded over the years but the constant among them all is the fact that each channel is 'discrete'; eg. a DD 5.1 soundtrack has six separate channels in the stream.
The engineers have the flexibility of putting different sounds in different speakers so that the audio is more realistic - like an airplane flying from left to right across the screen by placing that sound in the left, then center, then right speaker in a period of time. The Low Frequency Effects channel ('LFE' or .1) contains just very low bass and good bass adds to the realism.
When the receiver is sent a digital bitstream, it can manipulate it in more ways than when the input signal is analog. Features such as dynamic compression (aka 'late night' mode), cinema filter (tones down the treble), etc generally require a digital input signal.