Category is an indicator which Google understands and hence estimates the depth of the topic. If the category is relevant and is matching and is well fitting with the post and its topic. It becomes important for the author to select the right category which complements the content of the post. For Eg. if my post is related to latest happenings in World Economy I should select the category as Economy it is incomplete whereas if i select category as world, it is still an injustice to the topic. But if I put in the category as World Economy probably I am not only indicating clearly to Google of the content which is in Post but also opening way forward in enriching the users experience with new posts on this subject. Or may be I am able to place a content which is a follow up post or the way ahead of the previous post, or answer to the question raised by the previous post.
Thus with the category I am able to achieve the Google indicator marksman ship but also the way forwards for more posts on the subject, usually the category is the indicator of the depth of the subject and the count of posts on the subject indicates the richness of the blog. So now if you are looking for enriching user experience you have to work passionately in covering up the subjects raised by the topics which you raised on the subjects of previous posts. Now you have to not only make sure that the category is relevant but also it is in depth of the subject. If your category is shallow there is a brighter chance that you are stepping in shallow water and you wont be able to sustain longer with the subject. Thus if you want to be at the higher side with lots of content and follow up post options you have to be very alert while you are choosing the subject. Categories should be broader but should be relevant to the subject huge scope also invites higher expectation and you have to meet the expectations of the subject and also match the subject line and people sentiments. Because you have to always present the idea with few facts and research on the subject and this research actually shows your knowledge on the subject and opens gate for follow up mails and diversifiying post on the subject.
Now moving to the next subject References and Subject, this is very near to this post and seams to you the follow up post on the subject and hence if you are looking for more informations on the subject of improving Google Page ranking you must go through the next post which actually addresses the subject with different angle and also look at it from different angle. Coming back to the subject the category and its importance we have well understood now you have to look at the subject and the content research because a good category indicates content but the content quality is the only factor which makes your reader hooked up to your website. So keep writing good content and keep your readers waiting for your next post by mapping it with right URL String.