What is the point of a PhD thesis whose content already exists in published papers? In my area (computer science) it is standard to publish several papers during the course of your PhD and then only "write" your thesis in the last few months of graduate school --- often you only start compiling the thesis once you have secured a job for after graduation.

I realize that in other areas this is different --- your thesis is the primary output of your time in graduate school. However, in my area the primary output is the papers you have published and the thesis seems redundant.

Given that all the content of my PhD thesis already exists in published papers, what is the point of writing the thesis (other than to fulfill the university's requirements)? Why would someone (other than my committee) read my thesis?

I want to know what it is that I am trying to achieve when writing my thesis (as I am currently starting this process). Thus, an alternative question would be: How do I write a good/useful thesis, given that I have published all the results it will contain?

All of my papers have full versions on arxiv, which contain discussion of motivation, background, and related work, as well as all the details and some side results. (Unlike, say, mathematics papers, which tend to be very terse.) So in terms of exposition, I am not sure what would be added in my thesis.
Like @ff524 (comment above), I too have found PhD theses useful, and over the years I've collected quite a few of them. I've actually purchased at least 20 since 1992 from UMI/ProQuest. Unfortunately, I don't have access to their digital archive, but I have also photocopied more than double this number from various libraries during this same period. I have often found  them to contain fuller explanations and motivation, more historical details, and numerous minor tangential results, all of which tend to get edited out in the typical concisely written published papers (I'm in mathematics).

To answer your question, perhaps what you can do is to write an overview of your results and how they fit into the larger scheme of similar results on your topic that others have done. Even if no one else reads this, it can help by giving a useful summary of your work that you can siphon off when you need to give talks about your work (such as in job interviews) or when you need to write introductions or abstracts for future papers based on the work. Also, if for some reason you find yourself not working on this topic for a few years and thereby forgetting some of the details, and you want to return to the topic, your summary and overview will be very helpful in remembering what it was all about.