To remove moisture, air, atmospheric dirt, and abrasive metal wear particles from moving parts in the master cylinder and calipers that have entered the system. Brake fluid needs to be replaced periodically because brake fluid will absorb moisture over time which can lower the boiling point enough to make it boil at very low temperatures.  Moisture is very compressible which will give you a soft (spongy) pedal.  This will occur whether a vehicle has been driven 50,000 miles over the course of a year, or if the vehicle has been sitting in your garage for a year.  Brake fluid contamination is a function of time and humidity, and has nothing to do with the mileage driven or not driven. Moisture can enter the brake system through seals, connection points, calipers, the master cylinder reservoir, and through microscopic pores in the hoses. Please note that moisture can enter your brake fluid system every time the fluid reservoir is opened.  Please try to not open your brake fluid reservoir unnecessarily.

Vehicles with antilock braking systems (ABS) are even less tolerant of contaminated fluid and air than non-ABS vehicles. The ABS hydraulic pump operates at several thousand psi, forcing brake fluid through very small valves. This can whip air and brake fluid into something like the foam on a latte. This can make bleeding extremely difficult for an individual to do without the proper equipment. Those same valves and pump can be damaged by these tiny abrasive particles.

Air that has entered the ABS controller can be bled out but most likely requires the use of an expensive (and sometimes proprietary) ABS scan tool to cycle the pump and valves to purge the last of the air and foam.  G-Loc Brakes, LLC recommends consulting with a properly trained technician before bleeding your brakes.