A domain name will give you your own unique web address and you have the complete rights to it which can later be sold. It's not owned by someone else so you can include whatever content as you desire.
A good idea would be to have your own domain name and hosting, for example www.letterme.com or www.letterme.net and have a link to your blogspot blog on one of your webpages. This gives you more freedom to write any additional content which may not otherwise be so easy to do when you're restricted to simply a blogspot blog.
A unique domain name and content will also benefit you with google rankings and searches.