What was the significance of the decline of the Weimar Republic?
I think that the movement of the question to "Nazi Germany" underscores the fundamental significance of the decline of the Weimar Republic. The decline of the Weimar Republic made it clear that Germany was past the point of no return in its embrace of Nazism. The acceleration of the conflict that would become World War II is where the significance of the decline of the Weimar Republic becomes so prevalent. The lack of commitment to democratic notions of the good as well as the fundamental failure to address economic realities that confronted Germany is where the decline of the Weimar Republic becomes evident. The significance of this was to undermine democracy throughout Europe, reflecting a contingent view of the time in which democracy was viewed as incapable of dealing with the realities of the time period. In this, the significance is highly relevant. When the Weimar Republic declines, the hope of peace in Europe declines with it as Hitler's Nazism becomes the preferred choice of Germany and thus starts the movement towards the conflicts that underscore World War II.
The decline of the republic of Weimer led to the growth in power of the Nazi party. If the democratic parties had reunited to form a strong alliance (as they did in France) they might have won the elections and the Nazi party would not have gained power. Then again, these are all “ifs” that do not really matter. You could say though, that the decline of the republic of Weimer led indirectly to the Second World War.