Smoothness is different when talking about (realtime) rendering and actual videos.
In rendering each frame represents an exact point in time, which means (unless you apply artificial motion blur) there is no motion information whatsoever in a single frame.
In videography on the other hand each frame represents a timespan of the shutterspeed used. This means, even lower framerates can appear smooth in videos, because they contain some information about the motion during the exposure time.
The tradeoff is between smoothness and blur, a high shutterspeed means low blur, but a stuttering video, until you get to very high framerates. Lower shutterspeeds on the other hand introduce a lot of blur with high motion, but can appear smooth even with low framerates.
