Twin plugs make for a better burn and initial flame propegation across the top of the piston. It is especially functional on short stroke, large bore engines where an single plug on a multi-valve engine will have trouble with flame speed. Been used in Ferraris, Aston Martin DB4 GT and many others. In the modern era, it helps emmisions and keeps the converter in better shape by not dumping unburnt fuel out the exhaust valves.