mzhuk

It is because brake fluid, at least dot 3 or 4, is hydrophilic meaning the fluid will absorb water (moisture) from the air and no brake system is completely closed meaning this will occur overtime. As this occures the boiling point of the fluid and its incompressible characteristics decline meaning stopping performance and power suffers for you. If you take a look at the manufactures website of whatever brake fluid you are going to be using it will normally say it should be changed every few years (normally 2, but some recommend 1). Anyways once you change the fluid you will more likely have better response of the brakes and more pedal firmness assuming who ever does it does a complete flush of all the old fluid and bleeds the system.