I never use that function. Mainly because I usually seek rare files. So I might not be able to downld them one day but perhaps some weeks or months later I can get lucky with them.

If I clear downloads & incompletes I don't want, I do them individually (or perhaps select a few using SHIFT key.) It takes longer for me but as suggested I do like to keep those I'm in hope of getting some time in the future.

BTW if you ever preview files, LW creates preview files within the incomplete folder. After downlding these should be manually deleted. They will look like: PREVIEW-T-358**-title.ext etc.
To get around this, I launch the files from the incomplete folder in the LW Library window.