Why do some manufacturers reverse preamp polarity in the first place?

Several answers above as to why some preamp invert polarity.  
With last years big upgrades I can better than ever hear recording polarity. Lucky for me, my main preamp has a polarity switch easy at hand. Also with dipole speakers it is easy to hear a difference. Since reverse polarity the soundstage is larger and behind the speakers, with correct polarity the soundstage is more in front and between the speakers. Plus with correct polarity vocals are clearer. Bass is slightly more defined.  
There is not only correct and reverse. All sorts of angles.. Also some records multi tracked, some instruments one polarity some the other.. some 90 off due to running through gizmos.. One of the reasons plenty of Rock (in particular) recordings you cannot hear any real polarity... since in one recording it is all over the place.

We've had a polarity switch on our preamps since their inception. Changing speaker cables around is a real pain!

given so many reputed recordings are 'all over the place' regarding polarity/phase....

What then is the correct way to not adversely stress the power train when re ordering polarity?

mute the sound, then switch?

or just switch polarity on the fly as it won't matter for neither the speakers or amps will encounter undue strain or stress?

... just wondering

Much ado about polarity? Well....

Music by any polarity would sound as sweet? Nuh-uh. Scratch that.

A fool thinks polarity matters, but a wise man knows otherwise. Close, but...

There is no polarity either good or bad but thinking makes it so. That's it!

Most of the technical reasons don't hold water since it comes down to what the tap is labeled. A little dab of red will fix ya.  But a purist would still want control on a per-recording or per-track basis.