You need to sign in so that BlogBuilder and Warwick Blogs can tell who you are, what to show you and what you are allowed to do. Blog collections will be able to show you different entries based on your department, course of study or halls of residence even!
When you are browsing blogs, other people may set different privacy levels and, unless you are signed in, you may not see some of their more private entries.
You must be signed in to write and manage your blog with BlogBuilder. This provides a good level of security and prevents anyone else from writing entries on your blog, unless you specifically allow them to.