What is the point of this then? Tableau can directly connect to most of these data sources already. Is this meant as a replacement for a data warehouse to take transactional information into a format that is quicker to report on?

EDIT: After reading into this more it seems like Quicksight is Amazon's version of Tableau and connects to SPICE which is Amazon's version of a data warehouse. If you prefer Tableau, you can apparently connect that directly to SPICE. If you already have Tableau connected to a data warehouse, this would appear to be a new competitor to the market and wouldn't add anything to your specific setup (beyond Amazon's claims that they will do it better).

Tableau offers both desktop and server edition for data visualization. AWS QS also does data visualization but doesn't offer a desktop version. They are competitors. Once Amazon enters a market, game's over. It becomes a race to the bottom red ocean.