"The Google Updater is the program that downloads and installs all the software in the Google Pack. You can use the Google Updater to monitor the status of your installation, run software that's been installed, or uninstall software. A Google Updater icon will appear in your system tray and will display notifications when new software is available."
So if you try to download Google Desktop, you're forced to download Google Pack, or at least Google Pack's core, which is Google Updater. You can look at Google Updater like an equivalent of the "Add or remove programs" included in Windows. Unlike the application manager from Windows, Google Updater monitors the applications for new versions and is configured to automatically update the applications.
It seems that Google tries to centralize the way you install and manage Google software, so I wouldn't be surprised if you'll be forced to install the Updater for any Google software in the near future. Like Google Toolbar, which is a way to push new services and feature, Google Pack and the updater are a way to push new software from Google and its partners (there's a "Show new software" link that sends you to a page that lists software from Google Pack you don't have on your computer).
Note: In case you don't see the text below when you go to Google Desktop, this is the page where you should've been redirected. There's a workaround if you want to download only Google Desktop: this link.
Update: This is a new version of Google Pack (2.0 beta) that will be used to install individual Google products. Even if the download pages are not changed for everyone yet, Google Toolbar for IE and Google Earth for Windows will be installed in the same way.