Tumblr is not a blogging platform like Wordpress, it's much more a community like Reddit. Yes, you can use Tumblr to host your blog, but the majority of people use the dashboard to interact with others and use their blog to share their content. Very few people will ever see username.tumblr.com, they'll see the posts via tumblr.com/dashboard. Like Twitter, very few people visit twitter.com/citricsquid, they follow me and see my tweets in their stream.

For example, my dashboard right now: http://i.imgur.com/0YAYv.jpg (potentially nsfw material, didn't check)