Why do Germans celebrate St. Nicholas?

Were the shoes cleaned? And more importantly, were they filled with candy or charcoal the next morning? Many Australians shrug their shoulders, but in German culture, St. Nicholas Day is an integral part of the pre-Christmas period. But why does this custom even exist and did St. Nicholaus really exist?

Der Nikolaus, wie wir ihn am 06.12. jedes Jahr in Deutschland feiern, ist in Wirklichkeit eine Verschmelzung von zwei Nikoläusen aus dem dritten und sechsten Jahrhundert.