Commonly known for the phrase "Blood, toil, tears, and sweat" it was the first speech given by Churchill after becoming Prime Minister.
Prior to this, he was not the most popular candidate for the position and had a lot of opposites and most of British people was not so sure about supporting him. It's relevant because it wasn't until this speech that the majority of the parliament unified and trust his leadership towards the upcoming war.
Imagine.. What would be the outcome if the British people were not trusting him enough to follow his actions into the war? A divided people, government and army would represent another obstacle. To convince them may require addition effort and resources that could have been wasted or better used in benefit of the war cause. And a defeat of the British would have significant effects on the war results, Maybe the same significance as the France surrender.