According to the Ninja Website
Take your consumer camera to a truly Pro recording device. Avoid the internal MPEG compression & go from sub 30Mbps MPEG to 220Mbps ready to edit 10-bit 422 Apple ProRes HQ.
This External Recording article from Nikon states
Another useful optional accessory that is often used on more professional video productions is the external recorder. With an external recorder such as an Atomos Ninja attached to a Nikon HD-SLR, you can record a higher quality, uncompressed video signal from the camera directly to the external recorder via HDMI.
I'd like to understand what this means in layman terms? Will the image be sharper frame by frame? Is it like watching a higher FPS? Just more data to pull from when Grading? Is it useful on all video cameras or only high end ones?
If the final video is ultimately going to be uploaded to YouTube would it make a difference?