When I complete actions in my list manager, I delete them. However when I complete a project I mark it 'completed' so that it gets added to a list of completed projects. And yes, sometimes I complete a project that was on someday maybe or not even on a list, and I will move it to 'projects' and mark it completed. I love to hit that check mark and get that sweet sweet 'ding!'.
I've been doing this particular part of my practice for around 1.5 - 2 years and have hundreds of small and large completions I can scroll through for a boost in confidence, or to analyse my engagement with areas of focus or how my system has changed, or just to see what new ideas might be prompted.
After watching David's message yesterday I did some scrolling in this list and found a project which had been successful but which needs to be undertaken again. So I copied it along with the project support notes onto my active list. How cool!