I'm just trying to get an idea as to WHY it was wired that way. I need to wire mine in and I'm not sure how I should/would wire mine. Maybe I am overlooking some scenarios.
I can tell you why I do it that way. Mine is more experience based than theory based. I tried the theory and it did not work well. So I have changed based on actual experience. I am a long time 4-wheeler, and have done hundreds of winchings over the years.
My vehicle is setup with a dual battery isolater system. Winching is usually the option of last resort (tow strap is fast and easy), so if I am winching, I am stuck good. I winch with the batteries isolated, off the aux battery. This allows me to winch till battery is dead, and still have juice in the primary to start (or keep going) the vehicle . There is no alternator than can overcome heavy winching in real time. More than once, I have winched aux battery dead, and stop to re-charge aux battery and then winch again. That does not work if you winch is connected to your primary battery, IME.
I have also found, through actual measurement (V/CCA/RC), my aux is consistently better charged than the primary. The more stuff you hang off the primary, the weaker it is over time. So by using the aux, I am likely to have more winching power. Modern cars hang lots of stuff off the primary (alarms, computers, clocks, etc) from the factory, and end-users hang even more.
So this covers the major reasons for me.