You might think that bearings receive little attention in motorcycle racing, but this isn't the case. In production-based motorcycle racing wheel bearings are perhaps the most frequently changed. They may be replaced for a narrower version, reducing the contact area of the rolling element and, therefore, also reducing friction slightly.
While this may actually have a some direct benefit, it's generally done to create space so captive spacers can be fitted into the wheels. The downside is that the bearings won't last as long, but if they're being checked every 200 kilometers or so and replaced this doesn't matter much. And the cost is offset by the amount of time is saves during a wheel change.
Headstock bearings are sometimes changed too. In this case it's to create room to shift the steering stem within the headstock – moving it forwards or backwards or to change the steering angle.