The best way that I can think of to help you see how important the theme of your content is, is to show you how some people do this successfully and how others don't. A good place to see this in action is found on ezinearticles.com.
Now it really doesn't matter which category you choose. But since this training is about blogging, then you might want to look there… Go to the category called internet and online businesses. Scroll down to blogging and click on it. Over on the right side of the page, click top authors in blogging. You'll see on the page the names and pictures of their top 15 authors. What I want you to do is to start at the top, and look at the categories where each author has contributed.
What you can do is click on the name, and then scroll down past all the articles then on the bottom you'll see a list of all the different categories that they've contributed to. Most of the time you'll find that those categories will complement one another. But occasionally you're going to find people who have written in so many different categories that you can’t be sure what their theme is. That's a problem. If people can't recognize the primary theme of your content, then they won't know if you can help them. Do you see the connection?
Your blog is part of that content equation. In fact, it's the sum of everything else that you produce. The theme that permeates your content elsewhere on the web has to be consistent with the theme that you want on your blog. Not the other way around. It's your blog that defines the theme, and everything else must contribute directly to it.
You’re probably beginning to really understand why your blog lies at the center of your content marketing strategy. When people click on a link that takes them to an article or a guest blog post or an audio or a video, if that content is anywhere except on your blog, then it must enhance the expertise that you've shown on that blog.
If it doesn't, it will confuse people. And people who are confused go someplace else where they can find the answer to their problem more easily. They don't want to have to guess what the solution is. And they certainly don't want to have to think about whether or not you're the expert in the niche, never mind the one they need.