This isn't utilized in just Skyactiv engines. It is a method used in most newer makes and models.
The engine idle speed is raised to more rapidly warm the engine and catalytic converter. The grumbly sound heard in the exhaust note is from heavily retarded spark advance, which allows heated exhaust to be released into the catalyst before it is absorbed by the cylinder.
Retarded timing results in higher exhaust temps. Advanced timing will heat the cylinder more, as it will absorb heat from exhaust gasses which are retained longer. The upped idle speed is simply hurrying the process.
Also, DI engines are noisier than PI engine, as there is an additional fuel pump driven by a cam which is necessary to inject the fuel at the super high pressures, apart of the DI engine design. The injectors themselves are noisey as well, and have a clackity sound similar to a diesel engine.