There are two camps on this topic. One believes that keywords are all important. Why bother writing if people can't find your blog to read anyway. The other camp thinks that your content should pull in links and then readers. They say that writing for humans is the most important issue of blogging to begin with.
In truth keywords matter. Maybe not for the reasons that the one camp thinks, though.
Keyword research is nearly mystic. It is like you are reading the minds of your potential readers and it is free or really low cost.
Before search engines, there were focus groups. A fairly unreliable group of humans attempting to answer what they would do if.... What they said in front of the group was rarely what they did at home.
If you are writing any type of headline, on or offline, you have to do keyword research. Any headline should call for the attention of a target group. The it should be wrapped up in value to the reader.
The top keywords should not be your target. The long tails will have fewer searches each, but combined they will bring more traffic than the number one keyword.
The perfect way to write headlines for humans is to use niche phrases. They are more specific and; therefore, make a better headline. Better headlines lead to better content if you follow the headline. Always write the headline first.
All search engines want to reflect what is important to people. That is why the determine relevancy on links and anchor text.
That is the long and short of why keywords matter in post titles. You need to target an audience in order to be heard and read more often. You will get diggs, links, and more relevant search engine traffic.