A kiss is a very powerful motivational tool. To excite, to entice, to seduce. A kiss to different parts of the body can inflame desires. Can spur the feelings of the one being kissed so they feel more emotion, more adoration. To use it in a scene (depending on the scene) can provide gentle contrast to measured sadism.

Say for instance the submissive and yourself are engaging in face slapping. A gentle kiss to a reddened cheek could provide a nice contrast to the pain being inflicted. In the same manner you would rub the area where you cane/flog, when doing impact play to the submissives ass.