Yes. You can scan on demand what and when you want as needed. But usually if avast updates databases and protection screens are enabled, the computer is clean without on-demand scanning. If you run, for example, a full scan of such a system, then you can rarely find something. I run it only when installed on a computer where there was another antivirus. And in everyday life I launch very rarely.
With a resident (on-access) scanner the need for on-demand scans is much depreciated. For the most part dormant/inert files are being scanned, the other active files are going to be scanned by the resident shields when they are activated.
I rarely do an on-demand or smart scan other than usually as a result of a question in the forums.
But it is your system and choice, but having a scheduled scan is really no different to running it manually, it is essentially still on-demand.
From my experience. If all screens are turned on and antivirus databases are updated, then no on-demand scans are practically required. Usually I run a manual scan only if malicious activity is suspected, in other cases it usually does not give any interesting information because everything is clean. If there is an up-to-date avast on the computer, then I understand that it is pointless to scan it for viruses. Having avast, you often think less about viruses than about protecting legitimate applications from it and the glitches that it sometimes causes. ;D