I used to have bad experiences when installing Windows XP or the like - general inconsistencies between installs such as driver detection and automatic installation (or lack thereof), etc. But I haven't had any such issues with Vista or Windows 7.
When I reinstall, I just delete the partitions, run install and let it create them anew as required, then follow a pretty strict order for installing software:
1) Virtual CloneDrive. Whenever I get new drivers on CDs or anything, I rip the CD to an ISO so I can't lose it. VCD is a freaking fantastic virtual disk loader. Surpasses almost everything else out there (Daemon Tools, etc)
2) Drivers (if necessary - usually not, but my current mobo has proprietary network drivers so I need to install).
3) Antivirus. (When I'm cheap, MS Security Essentials - which is pretty much a top-rate anti malware package at present. Although when I remember to renew my license, I run Kaspersky Internet Security - as at present).
4) Windows Updates!! (this is the longest part.. the updating.. and restarting.. and updating.. and restarting.. and.. etc)
5) Nvidia drivers, and misc software (7zip, VLC, etc).
All up, that takes around 45-50 minutes on my current PC, from booting the setup DVD until completion and I'm loading Steam to play my games.
As for why I do it - well, you're right. Usually as long as nothing's wrong I don't reinstall all the time. I just had a bad month of different unrelated issues.
However I do do the 3-monthly install and the reason for that is just for cleanliness of the system.
As much as I hate this term, I'm what would be considered a "power user", like many of us here. A few weeks ago, for instance, I needed some new video editing software. I had several options, and I had to install and try 5 different packages before I decided on the one I wanted with the right balance of functionality and usability.
Now - in most cases, those other four (which I'm never going to use again) I have uninstalled but nevertheless they leave remnants on the system. Whether it be registry entries, folders, some third-party required software that was installed along with the package and not uninstalled on removal of the main package (examples, OpenAL gets installed along with many games, video codecs get installed along with video editing software, fonts get installed along with office suites, etc, and in most cases they don't get removed - these are just examples I'm sure you can think of others).
Multiply this out by all the different projects I work on in a week, a month, a quarter, etc. That's why I reinstall. To get rid of all that crap.
I'm aware there are programs which help keep track of what's installed along with software, specifically for this one bugbear, but that's not all it's about. It's just one aspect of my reasoning.