What are some examples?
I've gotten rid of almost everything I own twice by moving 7000 miles twice. I regret it.
Books, toys, trinkets, etc. I saw these, thought "I haven't opened these or touched them since I bought them". Turns out, there were memories associated with each one. Just seeing them would remind me of a time and a place. What I was doing, who my friends were, where I was, then I bought that thing. They all sat on bookshelves and I threw them away. I suppose I can go look at a picture of the shelves but it's not the same as glancing at them and having them trigger memories.
Tools, and supplies. I recently moved to a smaller apartment. I had a few boxes of things like left over parts from assembling furniture from 2 apartments previous. The day after I threw them out I needed them. All this stuff I have, 3 kinds of hole punchers, 4 kinds of tape (electrical, duct, masking, cellophane), a rarely need any of them except for maybe the cellophane. If I went by when "have I used it in the last 3 years I'd have thrown the first 3 away. But, that one day when I need it I'll be glad to have it. Same goes for various kinds of bandages, cooking utensils, boardgames, stationary supplies, gift wrapping supplies, ....
Another good example might be clothing. Yes, I do throw things away. But, there are various items I haven't used in years, long jeans for snow, high top hiking boots, thick socks, rash guards, etc.. that if I go by "have I used it recently" then I'd throw them out.
I'm not saying keep everything and no, no one will want this stuff. But, seeing both grandfathers basically have a maker-space full of stuff, and use it, makes me jealous that I didn't stick in one place and slowly build up my dragon horde of stuff too :P