a) Germany used a lot of iron ore from northern Sweden. The Norwegian port of Narvik was the main route the ore was transported to Germany. The North Sea is a more reliable water way than the Baltic Sea. To secure these shippings, Norway was to be occupied.
b) Securing Norway would open large parts of the North Sea to German shipping. In the First World War, the British blockade pretty much strangled all German shipping. Even military vessels were locked close to German waters. The Norwegian ports would allow Germany to operate bases far away from their home waters. (For the same reason all of the French northern and western shore were occupied). The Second World War saw German surface vessels attack allied shipping in the North Sea and the Arctic.
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