To put it simply, the primary circuit piston is the only one actually connected directly to the brake pedal. So, you push on the pedal, actuate the primary piston, which then builds some hydraulic pressure to actuate the secondary piston, at least until the initial spring force has been oer come. In a F/R split you'll find that the balance between the two return springs is altered to at least reduce the travel in the secondary piston vs the primary piston which therefore reduces pressure in the rear circuit and creates a bias, at least instantaneously during the apply due to hydraulic latency. This is probably enough to keep the rear brakes from locking first in a perfectly biased system. I know it sounds pretty minor.
