Line of sight smoothing processes the calculated path and removes any unnecessary points. For example: if point A can see point C directly, point B will be removed from the path. Then, if point A can see point D, point C will be removed as well. This process is repeated until the final node is reached.
The smoothing editor supports the application of multiple smoothing algorithms to the same NavTilePath, in case a custom smoothing algorithm is used. However, line of sight smoothing is absolute, so there is no benefit to applying it multiple times.