Not sure I'm answering this correctly, but I'll take a stab.
The output signal voltage level from the Twin preamp is VERY hot, line-level. Not a place for guitar pedals as they operate at "instrument level" and would severely distort and/or be damaged by such strong line signal voltage levels. So obviously the guitar pedal effects must live before the Twin preamp.
But another question I see is that of why didn't Jerry's guitar mounted master volume control the level between the preamp and power amp instead of the way he had it before the preamp. I see a few good reasons why.
1 - The Twin's preamp is a pretty high-impedance output and really doesn't want to drive long lengths of cable as too much cable capacitance could act as a low pass filter. The shortest connection from preamp to power amp is ideal.
2 - Sending such hot, line level signals to and from the guitar may cause cross-talk with the more sensitive, instrument level signals in the guitar.
3 - And mainly, the big one, the wiring logistics just don't allow for it. If the guitar has two cords, one a normal guitar cord, and one a mic-type cable with two inner conductors and a shield for the effects pedal send & return functions, there's really no good way to use the OBEL bypass switch that handles turning on and off the pedal effects while also sending the signal thru the preamp. Effectively, you would turn off or bypass the Twin preamp when you bypassed your effects pedals. The wiring scheme simply doesn't allow for it. The OBEL loop is instrument level only, just a pedal loop. To have an on-board master volume control that was patched between the preamp and power amp would take a whole other new 3-conductor cord, and at that point things are just getting messy and unnecessary. Basically, you can't have the pedal bypass function and master volume control on the guitar AND have that master volume control controlling the volume AFTER the Twin preamp without introducing yet another send/return cable to the guitar.
I can't see any advantage of a scheme other than Jerry's setup for what he wanted to best achieve. It's a smart and perfectly functional system evolved and developed by some of the brightest folks live audio ever knew.
Like Waldo said, the line level path after the preamp is the place for rack mounted line level devices like reverbs and delays, and then directly to the powr amp. What happens before the preamp is all instrument level and is the arena for guitar pedals and bypass loops and post effects pedal volume control.
... and it's just like any other day that's ever been...