Churchill recognized, as many of his Conservative colleagues did not, that Britain was engaged in a struggle for the very existence of its Empire. It's notable that the famous "blood, toil, tears, and sweat" speech was more enthusiastically received by Opposition MPs than by Churchill's own side. A fair number of Tory MPs were still ardent supporters of appeasement and didn't believe that Great Britain should get itself involved in any war. But toward the end of his speech, Churchill makes it abundantly clear that, in his judgement, the British Empire is at stake, and only victory at all costs will ensure its survival.
If Churchill had framed his speech in terms of a titanic struggle between good and evil, then it's unlikely that his inspiring words would've had anything like the same effect. But by bringing the fate of the British Empire—whose continued existence still enjoyed wide support—into the equation, he made it more difficult for his generally skeptical backbench MPs to oppose a full-scale commitment to war with Nazi Germany.
Churchill undoubtedly knew what he was doing by arguing that the British Empire was at stake; he hoped to play upon the instinctive affection of most British people for the Empire in trying to get as much support as possible for the enormous struggle ahead.