I think the talked about idea of scanning direction is basically the idea of getting an angle of return on the identified structure so as to be able to get the structure to show against the dark background of the water column...

Structure (such as brush) shows in an SI image as a brighter/lighter shade of the chose pallette color...if that brush is backdropped against the bottom contour showing in the SI image, it tends to partially blend in with the bottom...

Conversely, that same structure backdropped against the dark water column is going to stand out against that dark water column...making it much easier to interpret the structure's finer details that would disappear backdropped against the bottom...(this same idea is why some prefer DI over SI... the side facing aspect view or DI always puts the structure backdropped against a starkly contrasting background in the DI image)..

So as the guys have already said, sometimes it's beneficial to mark the structure and then scan it from different directions.... Maybe even scanning the structure in a direction that would allow that structure to be backdropped against the dark water column...

Rickie