Choosing a topic for the webinar is probably the most important part of organizing a webinar. Everything else builds on top of that. Your speaker, your content, and most importantly your ability to reach your audience.
Webinars are a great way to interact with your audience on a personal level. People attend webinars because they are looking to learn about a specific topic. By teaching them about this topic, you give them a lot of educational value, and build trust.