How can I use web analytics data to write better blog posts?

Analytics gives you an idea about what your potential readers are doing on your blog. It will show which posts are more popular with your readers and which are not. You can use analytics to get ideas for your future blog posts.

You will also come to know which sections are not performing well and hence work on them to make them better.

Web analytics serve only as guide, the main issue is for you to focus on your writing.  As I've said make your blogs interesting and up to date.  Use the active voice.