Re: Fly tying thread vs. Sewing thread. What's the difference?
Most sewing thread is 'spun'- there is as core that is wrapped by a secondary layer. It's also almost all polyester, and so is a lot of tying thread, so it would be colorfast (colors won't bleed).
The "old time" tying thread was mostly all SILK, like "Champion" and "Pearsall's" and in the 1960/70s, nylon thread hit the market under the name "Nymo". Danville's tying thread essentially replaced Champion as the primary product on the market in the 1970s, it was cheaper, available in multiple colors (Champion was in black and white only) and it was available in different weights. The other benefit to Danville's thread is it could be 'counter spun' and it would lay flat as you wrapped it, and where you wanted to, you could use a needle to 'split the thread' allowing you to insert dubbing and then spin it tight to make fine dubbed heads and bodies.
You can still find Danville, and if you look real hard, Champion... Nymo is all but gone, and Pearsall's is available from the UK still.
Almost everyone uses Uni-Thread, UTC, Gordon Griffith's or Bennichi now, and some use KEVLAR for saltwater and bass bugs. GSP (Gel Spun Polyester) is sort of popular and it's REAL CLOSE to sewing thread... I've tried it but don't care for it myself.
Does it matter? Who knows... I use Champion still for dress salmon flies, Uni for most everything else and Danville for larger flies... and I have some Bennichi for TEENY, TINY flies.