Hi lee,
You’ll probably get a better (and even more correct) technical reply from one of the team shortly, but in the meantime …
Unless you have the interface open, avast is “running” only in the sense that the resident scanners are active. If you want to run an on-demand disk scan, for example, or change settings other than for the resident scanners, you’ll have to start the “main” program, i.e. the interface.
Hopefully this will clarify a little what’s happening – I’m not usually very successful at explaining things. 
Best,
Mike