I’m going to assume that you know nothing about the brake system. If I’m correct then this is useful information. If I’m wrong then please accept my apologies.
So when you put your foot on the brake you are pushing the piston down the master cylinder which squeezes the brake fluid out of the cylinder and down a pipe which splits and goes to the brake calipers. The calipers also have cylinders and pistons. As the fluid enters the cylinders in the calipers it moves the piston in the caliper which pushes the brake pad against the disc to stop the car.
So when you bleed all four calipers which should be done in a sequence that is in the manual you will also be bleeding the master cylinder at the same time.
This should be done about once a year because the fluid is like a sponge and takes on water from the air via the reservoir. When the fluid gets to around 3% water it has a lower boiling point and you need to change it out because you don’t want it boiling when the brakes get hot. When the fluid boils it makes bubbles which can compress and then the brakes don’t work very well.
The brakes should also be bled after a day at the track.
Using Speed bleeder nipples makes the job a whole lot easier.