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The reason for the sandbox is to maybe catch new malware that is not listed in the current signature database. It will pick up on unknown apps that try to take actions common to what malware apps do. If you get an alert for something, and you don’t recognize the listed program and haven’t chosen to install it, you’d probably want to sandbox it. It’s just an extra layer of security that was added because the only knock on Avast! was that it was poor at detecting new threats.
If you leave the sandbox off, you’ll still be getting the level of protection you’ve been used to with Avast! and it may be enough. There’s always that small chance though that you could be hit by something new.