This is not an issue for users who want to install Dropbox on just one PC, but, if you want to download it once and then install it on multiple PCs, it makes no sense to download it individually on every PC. That web installer will download the full version of Dropbox to your PC and install it. When you click the download link, you get a small installer stub instead of the full-sized installer.
Like many products have started doing, Dropbox now offers a web-based installer for its Windows desktop client and also for Mac.