Hey Joerg Ben
Adding that title as a public aggregation to your control means that any user using your control can go ahead and add any entire sap.ui.core.Title with or without specified layout data. So if your control's name is MyCustomControl and they want to specify a title for it, they will do the following:
var customControl = new MyCustomControl();
customControl.setAggregation('title',
{
new sap.ui.core.Title({ext: "{i18n>title_test}"});
})