From b2evolution manual
Why are there already posts on my new blog?
You've just installed b2evo and you want to start clean. You know very well what blogs are, and you want to start your writing. But when you look at your site you realize that there are already four blogs in the package, with a lot of posts in them.
If your first impression is to delete everything you see, don't do it! Hold on a minute.
These posts are there for a reason. They are a quick-start manual. If you take a few minutes to read them b2evo will be a lot easier to understand.
After you read everything, you can delete those posts if you want to, but a better choice might be to deprecate them. That means you won't see them anymore on your blog pages, but they will still be accessible in the backoffice for later review.
What do I do with them?
The blogs that come with the standard installation are
Even if you think you don't need these blogs don't delete them. It's often a better idea to recycle them. You can do so by renaming them to something you want. This is usually more desirable than deleting them and creating new ones. You also have the choice of making these blogs invisible for the time being. You can rename and hide blogs in the backoffice under the Blogs tab.