What is the purpose of using riprap in drainage channels?

Riprap is an erosion-resistant ground cover made up of large, angular and loose stones (rock, concrete or other material) with geotextile or granular layer underneath. Riprap is commonly used in drainage channel to provide a stable lining to resist erosion by channel water. It is also used in channels where infiltration is intended but the velocity of flow is too large for vegetation.

A layer of geotextile is normally provided under riprap to perform separation from underlying soils. This prevents the migration of fined-grained soils from sub-grade into riprap and results in settlement and
loss of ground.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.