Well they help in providing a more stable spark and current transmission, which results in better and smoother running of the engine. If your current plugs and wires are in bad condition, then changing them will make you feel as if there is a lot of performance improvement, but in actuality it has just smoothed out the flow of current and spark and help your engine put out its peak hp. But this effect will wear off after some time when the plugs and wires become used.