His position was external and superior to parliament. .
Although Biz got the foreign policy position he desired, he knew that if he ever resolved the domestic policy dispute between the parliament and the king, Wilhelm would have no need to keep him around as prime minister or foreign minister. Fear of being dumped-fear of losing power-caused Biz to deliberately and continuously prevent a compromise between the king and the parliament on the Prussian officer corps issue. Until Biz won over the liberals in the parliament (who were also German nationalists) with his successful war against Austria in 1866, the parliament refused to pass the military budget and Biz continued to spend the money illegally regardless. After the Prussian defeat of Austria in 1866, Prussian liberals considered Biz a hero and even apologized for having resisted the military budget for the previous four years. The parliament passed the Indemnity Act of September 1866, which declared that Biz's previous unconstitutional expenditures, were legal, essentially nullifying their past assertions and accepting the Crown's actions in bypassing their authority. Now that the military policy crisis was over, would Wilhelm dismiss Biz? How could he dump a man seen as a hero? Furthermore, Wilhelm would need Biz to ensure that within the new North German Confederation, Prussia (aka Wilhelm) remained dominant. Certainly, Biz would be kept around. "Wilhelm I clung to Biz as the saviour of the Prussian monarchy.".
So, he came to power not because he had some grand vision of steering Prussia along a domestic policy path that would improve the economy and lives of all Prussians, but because he would be the conservative who the king would use to stem the tide of further democratization in Prussia. He became prime minister so that he might be the master, not of Prussian domestic policy, but of Prussian foreign policy.