Vlk from avast! said this on Wilders:
Absolutely. It's no mystery.
You have to realize that no AV will ever detect/block all binary malware files. Therefore, it's useful to use other protection layers - layers that are ideally independent of the scanning engine that's responsible for the detection/blocking of the binary malware (i.e. the overlap isn't too big).
Avast's Web Shield is an example of that. With 90+ per cent of today's malware coming from web sites, it makes perfect sense to put additional protection layers to the HTTP stack. In avast's case, we use an advanced javascript detection engine, combined with a sophisticated HTML analyzer and URL blocker -- and in practice, these modules are able to detect the vast majority of malware even without ever seeing the corresponding binaries.
Also, the WebShield allows us to track the sources (source URLs) of binaries that are later executed, and these sources are then taken into account by the heuristics engine (generally, files coming from high-profile sites are less likely to be malware than files that are coming from unknown/dodgy sites).
Thanks
Vlk
You can read the whole thread titled, "Web Scanning - Is It Needed?" here:
http://www.wilderssecurity.com/showthread.php?t=305375&highlight=scannerVlk has said on more then one occasion, that in avast!, the Web and File shields are NOT the same.