Why is formatting so important? How is it different to "Select all" - "Delete"?
Deleting: emptying all the drawers in a filing cabinet.
Formatting: installing a filing cabinet in the first place!
Unformatted card: empty, dark room. Try finding your paperwork in that!
Now think of fitting A3 sized paperwork into A4 sized drawers, and you'll get the idea behind different formatting options.
So if you always store A4 stuff in either A4 or A3 drawers, you're ok, but you could run into trouble if something doesn't fit one day. The good thing about formatting a card is that your filing cabinet is brand new again, no loose old paperwork or stuck drawers anymore, and if you format using the device that you want to use the card with, you'll be sure to get the right filing system
(Slightly low-tech explanation, apologies to any offended readers
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