Because Yahoo! Widgets supplements the Dashboard on OS X, not replaces it – with Yahoo! Widgets installed, OS X users are able to use not only Dashboard widgets in their Dashboard, but also Yahoo! Widgets: Yahoo! Widgets are platform-independent, and so different people are writing them.

Your question is like saying “Why buy a mobile when I already have a landline?” The two are for different purposes, and have different features and limitations. Right now, installing Yahoo! Widgets is the only way, as far as I know, to install an Aberystwyth Webcam Widget on your desktop (whether you’re using Windows or Mac OS), for example, so if you wanted one, you’d either have to install it or write another (both good options, but I’m sure you can see which one is easier).