It's immensely useful in a pinch, it's free form, and I can place it flat on a surface and write on it.
And, if I write sensitive information on a card, unlike a regular pocket notebook, I can store it or take a secure photo of it and physically pitch that index card.
Thanks, Merlin Mann[2].
I carry 2-3 3x5 cards folded in half, in my back pocket every single day. They form a small pouch to put small paper receipts into. Jot down shopping lists, notes or reminders. Of course, there's an ever present pen for this task.
I also carry a new package of cards (or two) in my bag. Just in case some idea or conversation or thinking or project comes up wherever I happen to be.
I don't understand how anyone can live without them.
I make a habit of spreading cards out over tables and surfaces when talking through a new or poorly understood subject. It helps clarify and organize and share and talk and think more than any other tool I've ever seen.
There is no digital facsimile that works anywhere nearly as well for people.
If you want to see what index-card-maxxing looks like, I present to you, the Hagoromo of Index Cards:
https://www.amazon.co.jp/-/en/Collected-C-531-Info-Cards-Plain/dp/B07GZMRNLS
No idea how to find them in the US, but I bought about 30 packs on my last trip to Japan and I've been sparingly using them for special projects.I configured my iPhone's action button to take notes. That means before I even take it out of the pocket or seeing anything on the screen that might be distracting, I can press and hold the button to allow me to type notes. I find typing faster than writing by hand and more efficient at capturing thoughts.
Of course I cannot physically rearrange them like you can index cards.
It's funny, I can feel a desire for a quick-draw-drawing competition!
I keep (only) my small folded cards in my right back pocket and my push-button pen in the left front (I'm left-handed) and there's a single, smooth motion of pulling the card(s) and the pen, clicking the pen and jotting something down. You're probably faster, but I'm not then switching to whatever feeds drag me in...
I've tried so many times and so many different tools. I've tried to make the tools too. There are dozens of tiny little quality of life things about small scraps of paper, but the biggest one is the re-arranging thing.
It's just not as easy to do with a screen.
Saw such a setup once where for each number was assigned a categorization different from the ordering, and for a given card, the matching hole would be opened up, then when searching a set of cards a rod would be inserted at the hole in question and lifted up and jostled, causing the matching cards to be revealed for inspection.
Unfortunately, didn't think to ask if was a product, or a mechanism devised by the owner's parent....
Fun to be reminded of them!
"Needle Card", "McBee Card", "Indecks", "E-Z Sort", "Cope-chat", "Keysort", "Flexisort", "Velom card", "Rocket card", "Slotted card" and variations on those all redirect to that article[1].
1: https://en.wikipedia.org/w/index.php?title=Special:WhatLinks...
Douglas Englebart apparently used them and had devised his own filing system. (I don't recall if that's discussed in this or another episode.)