Brake fluid absorbs moisture over time
Most motorcycle brake fluids pull moisture from the air. That lowers the boiling point and can contribute to corrosion inside the hydraulic system. Even a bike with low mileage can still need service if enough time has passed.
Feel changes often show up before a leak does
A softer lever, longer travel or inconsistent braking feel can point to old fluid, air in the system or a worn component. Those changes are worth checking even if the reservoir still looks full.
Use the motorcycle schedule as the baseline
The owner's manual should always be the starting point. From there, storage conditions, riding style and climate can justify earlier service than the bare minimum interval.
A brake-fluid visit is a good inspection point
Brake service is a useful time to check pads, rotors, calipers and hoses. That is especially helpful for riders who keep a bike stored through wet months and only bring it out when the weather improves.