There are still some things Dashboard can do that can't be done elsewhere, like display a part of a webpage without having to go to it in the browser. For this reason I still use it sometimes, and since it can be disabled in Yosemite there's no harm in leaving it in (although I'm surprised they did so far in 10.11).Honestly, I never found any use for the dashboard. Maybe it should be removed or replaced by the siri window in iOS 9 when maybe siri launches on mac.
There are still some things Dashboard can do that can't be done elsewhere, like display a part of a webpage without having to go to it in the browser. For this reason I still use it sometimes, and since it can be disabled in Yosemite there's no harm in leaving it in (although I'm surprised they did so far in 10.11).