The primary reasons for surfaceships raiding the oceans for Germany was to force the Allies to allocate mresources to counter this threat, making these same resources not available for other uses, such as in the Mediteranean Sea and ASW as well. (Cruisers and battleships at sea also need escorts against the ever present Submarine threat.)
The U-Boote naturally were the main item against the Allied Supply convoy's at sea, but the eight in all HSK's and the occasional cruiser too forced the British to formulate answers to this, by distributing enough navalpower all over the oceans (Whole Atlantic and Indian Ocean and partly the Pacific). These forces therefore were badly missed in the European theater of war, especially in the Med. The absense of especially aircraft carriers, which were hold back mostly in the UK to counter any German large warship breakout, was felt most, since there always were too few of these.