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Most infections today are drive by download or java exploit. If a site is infected by a java exploit your java plug-in in your browser downloads other malware if your antivirus don't detect it.
A browser used with Sandboxie for instance can't harm anything of your real machine. All bad things aren't able to bypass the sandbox. A far as I know. Therefor sandboxing is useful in my opinion.
In this topic we are talking about the auto-sandbox feature here.
You can't elect to run a browser sandboxed in the auto-sandbox. For that you have to be using avast Pro or AIS versions with the full sandbox feature.