Why was the first battle of the marne considered so significant?

Because before the battle the Germans were very close to Paris (in its outskirts). To be a little contractual if the Germans would have won, they would have probably entered Paris and a general collapse of the French Republic and its army would bring to total collapse of the western front. Considering that the English did not have the capability to face the Germans alone the war in the West would has been won by the Central Powers.

The First Battle of Marne also known as the miracle of Marne halted the impetuous German advance and assured, in a way, the Antante victory for years later.