In terms of BDSM play, many dominants use over the knee spankings to dominate their submissive as it humiliates the submissive and allows the dominant full control. Spanking over the knee also prevents the spanker from 'mis-firing' and hitting the lower back or thighs.
More About Over The Knee Spanking (OTK Spanking)
For a dominant, an over knee spanking is great because it leaves the submissive exposed in many ways, they get a great view of the submissive's behind and allows them to build tension as the submissive will not know when they will next be punished. For the submissive, over the knee is quite a comfortable position and still allows you to be submissive if you don't enjoy being tied up.