Why? or how often? Reason for the why is after it does an initial full system scan, and given that it's constantly on guard, wouldn't a full scan be kind of pointless, as it should have dealt with any insurrection from the onset, and shouldn't let in any malware while it is running.
Why- sometimes malware gets into places not always accessed on a daily basis. It could have gotten there before it was added to avast's detections.
How often-depends on the individual user, what they use their computer for ie downloads, trial software, shadey sites, etc. Some do once it a week. Since I don't go anywhere, it's once a month.
@ianeuropean
No scheduler in the free version. DavidR has posted a few times a way of using task scheduler. A search of this forum with the search key words, without the quotes, turned up this, " schedule scan DavidR" among others others
http://forum.avast.com/index.php?topic=30615.0