Your base should be engineered to shed or drain water appropriately, especially in wetter climates. Assuming your base is in good shape, adding fibers increases the saturation point of your footing versus sand alone. This enables higher water absorption, which delays saturation that causes your surface to get too loose and slippery.
A traditional compacted base will be either crowned or sloped in order to shed water across its surface, whereas a free draining base is designed for the water to drain down through the base and can be carried away via drainage pipes below the surface. There are pros and cons to both designs, which we will dive into in another post.