Many blogs display the same boring templates or skins for the layout. While the template does not affect readers who enjoy your blog via RSS feed (unless they click through to the blog), but it really affects any other blog visitor who reads the blog from the web browser.
The more unique looking a blog design is, the easier it is to build brand recognition and a good name that gets you into viral marketing. If your blog looks like dozens of other blogs, your own blog will not stick out from the crowd and you will lose out on visitors. In the past I have been using free WordPress templates myself. I applied minor modifications, but it never really separated my blog from others enough to be really unique.
The problem with so many blogs is to find out what is their main specialty. What is the main topic a blogger is writing about? Often there is no way to tell what a blog is about – especially if you enter a blog from an older posting through a search engine listing.
The homepage might eventually have a small snippet of text explaining the topic of a blog, but archived articles do not show this normally. Or the blog owner does not even bother at all to tell what his main intent for the blog is. By using a custom blog design this can easily be integrated and helps to make the blog look different from anyone else.
By using a custom header as an example it helps to get the unique look you are looking for. First of all if it is completely unique it makes my blog stick out more. Second it can help you to show what your blog is all about and therefore get people to come back.
Make your Blog look unique + make sure your visitors know what your blog is about. Just because they find a (one) posting of your blog that matches their search term, it does not mean that your blog is really of interest to them. But you might be able to catch their interest by pointing out what your blog is really about. If you catch their interest you might gain a new reader that a) comes back to read more from your blog and b) eventually tells others about your great blog.