Sometimes you need to be able to do an 'outer join' against sparse data. Like if you have a few dates and you want to lay those across a calendar, but you want to color code a mark for every single day. Or if you have a bunch of potential output categories but your viz doesn't currently show them all, and you want to have a category displayed for each one.
If there's a sequence not represented here, please send me an email at Hermann.Chong@tableau.com