Just as you regularly change the oil in your engine, the oil in your suspension needs to be changed as well. Worse yet, your suspension doesn't have a filter!
With each stroke of travel, the polished surfaces of your suspension pick up microscopic particles of debris. Those particles get drawn into the oil, and over time contaminate the interior passageways. As the miles accumulate, so does the grunge. Without regular service, your high-performance suspension begins to lose it's ability to dampen force.
To keep your investment performing at it's design level, Ohlins recommends servicing your street motorcycle suspension every 12,000 to 15,000 miles, and your track/race bike suspension every 10 hours of track time.