A live audio feed "is" a scanner (or another type of radio) streamed to the internet. A scanner/radio can only receive within about a 30 mile radius. Feeds allow listeners to monitor radio traffic from areas far, far away from their listening area.
If listening to a local feed, one is restricted to what is on the feed. Using a scanner you can monitor more nearby systems/frequencies then what are provided on a feed. Also most feeds do not provide police/fire tactical and investigative channels as well as ambulance-to-hospital transmissions.
There are probably hundreds of channels to monitor on your scanner that are not streamed.