Final answer:
A speech aims to inform, persuade, or entertain, and its success depends on understanding the audience and the rhetorical situation, as demonstrated by Johnson's and Roosevelt's speeches.
Step-by-step explanation:
A speech can serve various purposes such as to inform, persuade, or entertain an audience. The effectiveness of a speech heavily relies on the speaker's understanding of the audience and the rhetorical situation. For instance, Lyndon B. Johnson's speech aimed to persuade the American public to support his Great Society proposal.
Similarly, Franklin D. Roosevelt's Pearl Harbor speech had a dual function: to inform Congress of the attack and to persuade them to declare war. The alignment of the speaker's purpose with the audience's values and the occasion is crucial for delivering a successful speech. This involves choosing the appropriate tone, language, and delivery method, whether it's through various media channels or a presentation to a live audience.