First, When we use a blogspot URL our audience may find difficult to remember(sometimes).
Why do they feel difficult to remember?
My custom domain URL is :- http://www.krishnainfotron.com/
My blogspot domain URL is :- http://www.krishnainfotron-l.blogspot.com/
When I am on a custom domain, It eliminates the user to remember blogspot in the middle. Remembering a word may make our visitors frustrate that “Ah! How can I remember such a big/lengthy URL?” ( :P )
Second, due to blogspot’s CC TLD aliases, when you are on a custom domain, people won’t get worried/confused/tensed what the blog URL is actually?
Third, You can have your own mail ID like firstname.lastname@example.org where, you can manually gain traffic to your blog.
When you share your domain mail ID in your resume, surveys and etc., people see yourdomain.com as a different mail provider and they try going to the URL. This really increases the traffic.
Fourth, Simply you can tell people that you have your own site. :P