RejZor,
Simple answer, you find something lurking on your hard drive with a online scanner (Bit defender) that Avast missed with default settings!
After that experience surley it is only commonsense to take extra precautions 
The decision is all about risk and potential threat. avast effectively has two types of scan on-access and on-demand.
The on-access resident scanner can be set to high or custom and can provide protection against newly created/modified files no matter what the file type. So do you need to have the on-demand scan repeat that exercise.
Archive files (not self-extracting .exe, which different and would be scanned) are by there nature inert until you open, then execute a file within them (two steps). When you open a zip file the resident scanner depending on settings should kick in, if not then, when you try to execute a file within. So there are several levels of detection/protection.
So if you do a thorough scan with archives included when you first get avast, and maintain a High or Custom level for on-access, new files should be scanned. This lessen the need for regular thorough, archive scans. I only do a weekly standard scan excluding archives and I don't feel overly at risk. Part of this can be down to not downloading all and sundry from the Internet or opening unknown attachments, basic common sense.
