Royal nomenclature in the German case was deeply entwined in politics. When Germany unified in 1871, the exact title of the state's monarch was a subject of contention. The Hohenzollerns were already hereditary kings of Prussia and would never give up that title until Wilhelm II's abdication. In fact, the German constitution legally connected the title of Kaiser to the Prussian crown. The imperial title was needed because the the federation of the German empire also had three hereditary kingdoms within it, such as the Wittelsbach of Bavaria and Wettins of Saxony. Some of these royal families had a greater heritage than the Hohenzollerns, which Wilhelm I's Wittlelsbach contemporary, Ludwig II, was fond of reminding him.

In general, European royal families will seldom give up the title of king once given to them. The initial title of the Hohenzollerns was King in Prussia, but under Frederick William I and Frederick II dropped the "in" for "of" as a sign of their independence. Thus for a royal figure to command superiority over his other royal subjects, be they kings or grand dukes, he needed an imperial title.

This question has been removed as it is not about history but about linguistic usage.