A dimpled drainage mat would allow water to move on top of the soil under the mat, when the soil is saturated, towards an area to drain like a sump pit. If you have no standing water and soil that will drain, and no need for a sump pit, then there should be no need for a drainage mat. Besides, if you needed a sump pit because of lots of ground water, you can achieve drainage of the entire crawlspace with some perforated pipes leading to the pit.