If the cheaper 14guage wire was used in the walls then only the 15 amp socket is appropriate, otherwise you might plug in a 20amp device and melt the wires or burn down the house.
Not really. 15A receptacles are required by code to be able to handle 20A of current at their terminals, so the larger wire and breaker allows for more loads to be run instead of one big load to draw it all (and if you need that, then a 20A circuit with a single receptacle does actually require it to be a 20A receptacle).