As brake pads wear the fluid moves from the master cylinder to the calipers. When pads are replaced the fluid moves back. As long as the fluid level is adequate, I usually don't add any as to prevent overflow when brake pads are replaced.
There is usually a fluid-level sensor in the master cylinder. When the car is NEW, the fluid is at the MAX level. As the pads wear, the level drops. So if the BRAKE warning light comes on, it is an additional indicator that "It's Time for Brakes". (That is in addition to the 'squealers' built into the pads that make rude noises when the pads get thin.)
If you do the 3-year recommended brake fluid flush, fill the reservoir to the same level as when you found it. Otherwise, listen for the squeal.