Pretty much what the xkcd says. I basically never delete files, they just get moved to deeper layers as time goes by. I roughly know where to find my elementary school presentations, first websites, university assignments and emails, rotated irclogs.. The mental map will take me there if curiosity awakens, better not to mess it up by organising.
https://xkcd.com/1360/
I once found some x86 assembly code I'd written decades ago and buried real deep (the folder names were in ALLCAPS and no more than 8 characters.