Just because you have a wordpress blog, that does not equal that you don’t want to have registered users that can for instance be blog publishers. Also, there are many other kind of wordpress websites besides blogs, communities, commercial and sales, forums, etc.

In all these occasions you need to allow user registrations. Easy to accomplish, just navigate to your website backend admin, Settings -> General

Make sure you tick:

 Anyone can register

You might also want to show the admin toolbar in the frontend of your website to all users, including guests. By default the admin toolbar does not show, but there is an easy way to bring it to life, just follow this how-to.