Headline tags play immense importance in your blog or site optimization to increase the chance to score higher in search engines.
In Blogger, by default all blogger themes are not search engine optimized and they are designed such that the blog main title tag is <h1>, description of the blog and header dates are with <h2> title tags, sidebar headers and post titles are assigned with <h3> title tags.
The fact is that in most cases the post titles tags are second most important part of the blog after the main title of your blog. Post description and sidebar headers are less important in comparison to your SEO post titles.