For one, here's the use case that inspired the project: SmartyStreets uses it so people can process their address lists. It was difficult for some customers to upload their files full of personal data either because of regulations or just being concerned about privacy... so now they can just do it all in their browser.

Scientific and research applications would find this useful... any web app dealing with tabular data could benefit I think. Even if you still have to upload the whole file to a server, being able to instantly render a preview is a big win.