Explain why "narrowing your topic" is important to the public speaker. Be descriptive.

You're to narrow your topic down, and shave off the details in a speed, so that you fulfill your rhetorical purpose. It allows you to fit your speech into the available time and helps you to focus on the important talking points.

Narrowing your topic is important because it allows you to fit your speech into the available time and helps you to focus your speech.

Stay away from these ideas:

"Prevents you from choosing too specific a topic" is an incorrect answer because you want your topic to be relatively focused, and "helps you to brainstorm additional ideas" is an incorrect answer because you engage in brainstorming to select a topic, not to narrow it down.

"Captures in a single sentence the overall message you want to convey in your speech" is an incorrect answer because it describes a thesis statement, not the process of narrowing down the topic.

"Eliminates the need to prepare a specific purpose statement" is an incorrect answer because your specific purpose statement is written after you have narrowed down your topic