I can help some I think.

Motion blur is used primarily for graphics. Since most graphics are built frame by frame, they tend to have sharp edges and very definite movement. Sometimes adding a little motion blur creates more realism. This can be illustrated by looking at some of the early works of Ray Harryhausen. Using stop motion animation, he made very cool dinosaurs and such, but every frame and movement was very sharp compared to video around it. Whereas, on a movie like the second Raiders of the Lost Ark movie, they used stop motion for the mineshaft chase scene, but by adding motion blur, realism was added and most people didn't know that scene was mostly animation. (they actually added the motion blur while shooting by moving the camera during each fram exposure but the effect is similar)

Gaussian blur and other blurs are sometimes used to take the "edge" off of video. Sometimes video is just too sharp. People are always striving for that "film" look and, previously, part of that look was softer edges around objects. Even now, film will always appear a bit more out of focus than video, even though technically it isn't, just because of the nature of film vs. videotape.

I personally like to use blur for effects such as title role on the end of a project. Or for a background for graphics, throw some live footage in a blur and super the type over. I also use blur sometimes for transitions between scenes just to break the monotony of a simple dissolve.

Hope my non-technical explanation helps some.

Dave T2

Sometimes for effect, but most often a little (1-2 pixel) linear blur in the vertical position can reduce or remove interlace flicker on stills or graphics.

Dan

Also useful with keyframes to simulate a "pull-focus" effect.

(i.e., object starts out of focus and gradually resolves into focus)