The "Smoothing" method uses a moving average of the image luminance computed from a number of frames "ahead of" and "behind" the current frame. If there is no preview, the moving average is only for frames behind the current frame.
A preview is not absolutely necessary, but you may get a smoother result with a preview. A RAM preview works just as well.
You specify the number of frames before and after the current frame in the to use in the smoothing method’s moving average. When the clip is previewed for the first time, only the values before are used in the moving average. After the clip has been previewed, the values before and after are used in the average. For this reason, a smoother output can be achieved by previewing the entire clip before doing the final render