Oil and other fats carry flavors well, especially fat-soluble flavors. It also adds shine, as Larry notes, and can reduce the amount of moisture lost due to evaporation. Imo, it works best if it is emulsified and for this I use prepared mustard in my finishing sauce. Mine is vinegar-based, with Worce, Dijon, salt and a little heat and sweet. A water addition cuts the acidity. It emulsifies well and this allows it to cling well to the meat, not pool in the pan. I use this as a dipping sauce for ribs but as a finishing sauce for PP, just 1 or 2 T per cup or so of pork.
Finishing sauces don't often taste like much on their own but round out flavors and textures when used as intended.