Blogging with one category is best for your blog rank and traffic. It is very helpful in SEO for putting your blog's post in appropriate keywords. The visitors who are viewing your content is going for the post on which your blog is categorize for, then good but when a visitor want more categories content then i don't think that it is possible for a person for write the content above his category. Therefore, to reduce such problem these blog network were introduced.
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In a Blog network, many bloggers comes together and grabs posting in each category, This attracts the visitors of different interests, which too give your blog many new users.
Many times, blogging network helps blogger to earn a huge money. This actually happens when the bloggers gives their advertising place to the Blog Network and blog network takes the benefits of it and ask their advertisers for putting the Ads in package on Different Blogs.
The Bloggers gets a good reputation in a Blog Network and a huge number of revenue is generated can be from it.
Example of a Network (blog):
Problogger is in the Blog Network of b5media.com .