Back when I had a P55-UD6 I wanted to add a switch to BD-PROCHOT so the board would boot with lowest CPU ratio when set and basically got that answer when asking Gigabyte support where the signal came out. Understandable answer but annoying. Had to teardown the cooling, remove the CPU and connect to the socket pin with one end of a meter while using the other one on likely components. Found it and switch worked beautifully. They could have saved me the trouble to look for but I guess it wasn't worth their time. That was just asking about a specific point, not for a full blown schematic.
Schematics is not just about repairs.
On my GTX1050Ti I was also able to calculate and add just one surface mount resistor to achieve a 20% boost in GPU voltage, so at 1.000V set it would actually be 1.200V, just a little over 1.3V on max. Well it actually measured on a meter at +18%, good enough.
On my x99 I was able to see there were 2 free inputs to the super IO so was able to add my own temperature sensor used to measure Tcase and have the reading read in software and be in sync with all other readings.
'Boardview' can be really helpful too.