The German people were very much concerned with the concept of the "Volk," (the people) for some time. The Volk was not separable from the German state. Anyone who was not a member of the "volk" was automatically inferior. The idea existed long before Hitler came into his own. As a child, Hitler was an avid reader of a pornographic work entitled Ostara, which was blatently anti-Semitic. Hitler's favorite composer was Richard Wagner, who was also a rabid anti-Semite. If one reads Mein Kampf, Hitler argues that the Aryan race is by nature superior to others. The idea did not originate with Hitler. Having said that, I should mention that the German concept of untermention was not directed solely at the Jews; the Nazis had no use for Slavic people or anyone who was not of Nordic descent. A little known fact is that when Jesse Owens won the Decathalon in the 1936 Olympics and was presented to the platform; Hitler refused to shake hands with him. He later commented to his aides that the Americans had basically cheated by bringing a "trained animal" to compete.
Hitler did not "invent" racism; he only became the chief spokesman for an idea that was already there. An excellent book on the subject is The Psychopathic God by R.G.L. Waite.
From Hitler's perspective in order for his actions with regard to the 'Final Solution' he had to first justify them. Ironically, Hitler's prestigious SS began to show signs of demoralization as a result of their daily execution of hundreds of Jews by firing squads. In became clear to Hitler and other high ranking Nazis that their policy of execution had to change. Now, it wasn't just about the race of Jews, it was about the dehumanization of the Jews. The Jewish population had to be reduced to 'things' or 'animals' as to ease the plight of those charged with carrying out the executions. That was when the concentration camp was born. An industrial like place where by 'animal' like things could be discarded...not human beings. Hitler's genocide began with race but in order to withstand the horror perpetuated the justification came by dehumanizing a portion of humanity so that the SS could perhaps sleep better...something to think about
It was Hitler's intent to make the German's a "pure" race. In order to do this he had to eradicate the "Jews". In his eyes the Jews were not pure and were the root of all Germany's problems.
There are many ways to answer this question, depending on the level at which you are asking it.
At one level, the Nazis believed race was important because they believed (or at least their leaders professed to believe) that the Germanic races were superior to others. They believed that Germanic people could create good societies while other types of races could only tear down societies made by the Germanic people. In this sense, then, race was important because their country needed to achieve racial purity to avoid being torn down and made weak.
At another level, though, you have to ask why the Nazis were willing to believe this. That is a much harder question to answer -- why would one group come to believe that they are so superior to another. One answer for this is that it was a defense mechanism. The Germans were so unhappy about having lost WWI and about the Treaty of Versailles that they were glad to point at scapegoats. They wanted some way to prove that their problems were not their fault and it was convenient to blame them on people of "other races" like the Jews.