Yvan Fornes says:
This approach can be useful when you need to visualise marks that are on top of each other.
It is a style similar to an area chart, but the data is divided up by calculated ranges within the data, rather than dimension members themselves.
A comprehensive tutorial is available on Tableau.com. I think you need an account there to view it:
Nathan Yau explains the use case for this chart type:
Joe’s inspirational viz is available here: