For example, the knee pads pictured below are a thin foam rubber. Because they are thin, they do not provide adequate padding or insulation from a cold ground.
In addition, when immersed and compressed in water, they absorb water and retain it. This makes for continually cold, wet knees. Not good.
This Instructable demonstrates how to make inexpensive knee pads that provide more cushioning and insulation than these cheap knee pads. Most importantly, these new knee pads do not retain water.