Dear @DavidR
I can explain it to you roughly. I use one of my external HDDs mainly as a storage swap, which Avast can ignore safely.
Now and then, however, it happens for space and psychological reasons that I use this HDD as temporary storage for particularly large downloads. Here I would like to perform a targeted Explorer scan if necessary, without having to manipulate the exception list beforehand.
That is the rough explanation. But there are other reasons why such a function would make sense (in my case).
I hope that helps a bit.
Best regards
Michael