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For the same reasons movie studios release trailers and teasers before the movie hits the cinema. To make people aware of this really cool thing that will be coming soon. To get people talking and interested. So that when it is ready a lot more people will know exactly what it is and be ready and waiting to use it.
Also this being an open source project the more people that get to play with this the greater chance they have of getting developers, testers, comments and bug reports which will speed up and improve development.