Cancel To Order, New And Used

I recently received a large box of stamps from a great grandparent. When I opened the box, it looked like a box of confetti! I spent 6 weeks sorting them by country and stamp and now I'm cataloging them using Scotts 2017 catalog. I have fallen in love with these stamps.
My stamps are from dead countries and the commonwealth countries. Most of my stamps are cancelled but they appear to be unused as far as the gum on the back. How can that be?
They are not on a cover and cancel to order stamps started in later 1900's. I should consider them to be used but how does this happen?
Almost all of them are singles. I thought cto stamps only came in sheets.
I'm impressed that my great grandfather was able to find all these stamps with just pen and paper and maybe some long-distance phone calls.
Talk about patience.

Thank you for your time

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Cancelled to order comes in different flavors.

What I think you are talking about are issues that were sold at face but cancelled neatly. For stamps Before World War I, Serbia and Australia comes to mind among many others. Many collectors long ago preferred used stamps over mint , believe it or not. And this type of CTO is still available for collectors today. You might show us a couple of examples of what you have to confirm suspicions.