The same reason at some point you will have to change your brake fluid when it goes from the clear it originally was to brown/black. The fluid breaks down and affects performance, maybe not in a huge way but more for piece of mind. Moisture over time will build up in the system or you may have some air in the system. Just go get a decent size container of brake fluid, whatever brand it may be. Use the speed bleeder or get a friend to help and do the pump/fill method until the fluid comes out clean and clear. Cake job.