different spring seats for SS springs?

1wild&crazyguy said

no perch change, though if/when you find out the pinion angle is crap...then the new perches will come in handy for the cut'n'roll.

Click to expand...

so i understand i can use my stock seats...and yes pinion angle will be changed, ordered some shims and they are wrong, so i might just be redoing the seats

but physically, what the heck is the difference between these two different part numbers for spring seats? if the ss springs work with stock seats, why are there seats made for SS springs?