I have learned to save all of that stuff. My wife collects antiques and she watches some of the shows on TV where they buy/sell vintage items. Through all of that I learned that items with the boxes can be worth more so I decided to start saving all of my boxes. But there is a downside to this in that for most items that we deal with the increase in value is a small percentage and keeping the boxes is a lot of trouble. I moved recently and I had original packaging for flashlights, cellphones, computers, whatever. So I might be overdoing it a bit.
As for knives, when I buy knives on the forum I am usually buying them to use so I don't care if they come with boxes. I would actually prefer not to get boxes and pay a "user grade" price for the knives. Then I don't have to worry about keeping the boxes and when I get ready to sell the knife I sell it as a used knife and don't have to worry about the boxes. I do have a knife that I bought about 15 years ago to use as an EDC but I never carried it, now I find that it might be a rare model and I was considering selling it, but I can't find the box. I might not have had the box or I might have thrown it out, but it is harder to sell it now as a rare collector if it doesn't have the box.