A small preference.

[Ouzel]

I was wondering why some mgfs. put out 'waxed thread'.

Unwaxed thread is less stiff and will lay flater on the hook or material, if you're dubbing the tiny bristles of the thread will provide a better surface to grab the material and for the same reason unwaxed thread will absorb and hold head cement better.

Maybe I'm being obtuse, but I don't understand why a mfg. would add an extra step to the process and thereby increase their costs.

Unless of course they make the thread for other uses and thought they would increase its marketability by offering it to fly tyers.

I do know if you are tying larger nymphs and streamers and need body build-up, waxed thread is the way to go but that single reason seems insufficent to me to add a mfg. step.

A small thing and just wondering 'why'.

[Strike II]

I believe the wax adds some addhesive properties to the thread, thus making it grab and hold materials better.

[Ouzel]

Point taken, as I have used 'tacky' wax in prep. for dubbing and also the final whip finish but I have not found 'prewaxed' threads to be tacky.
If they were the spools might accumulate fuzzy stuff from the bench etc. and wind up looking like hair balls.
Maybe they just put a very light coating.

[carpman]

Wax can also help "lubricate" the thread. This sometimes adds strength and also helps the thread cinch down. For instance, when ending a whip finish the wax keeps the thread from cutting into itself................

[Ouzel]

they use a different type of wax when coating a thread then the type I apply (Loons) when dubbing or whip finishing.
My main objection to pre-waxed threads is that I've found it stiffens them and makes tying, for me, a little more difficult, and more so with smaller hooks.