First off since I see this is your first post, welcome to the forum!
Brake fluid needs to be changed because over time it gets "wet", meaning that water gets absorbed into the system. When that happens, as brakes get hot, the water will boil at a relatively low temperature compared to pure brake fluid. That causes water to turn to vapor, which (unlike liquid) is compressible. The problem with that is that when you press the brake pedal, you want all of your foot's force to be transmitted straight to the caliper, which requires that nothing between your foot and the caliper be compressible. But when you have compressible gas in there, some of your force is lost compressing said gas, which means that the more gas in the system (caused by more water in the system), the more force from your foot will be required to achieve a given force at the caliper. Incidentally, this is why performance brake fluids exist; their fluid has a higher boiling point than standard fluid (and MUCH higher than water), which means they can remain in liquid form and transmit all of your foot's force to the brakes even as the brakes get much hotter.
Notice, though, that none of this really matters until you really heat up your brakes, so if you're just driving rather moderately, you can definitely hold off replacing brake fluid a bit longer. In fact on cars produced 3/2011 and later, the service indicator is programmed to require brake fluid changes every 3 years rather than 2, even though there was no change in the braking system or the fluid; it's just BMW cutting costs on Ultimate Service. So you'll be fine!
