It's to show saturation and how that saturation might look in different nibs without using, well...different nibs. Not to mention, it also shows shading characteristics and "true color" that might not show up in the review scan.
All I do is take a Q-tip and dip it into the ink lightly for that first stroke. The second line is two strokes; the third, three, and so on. Scanners aren't perfect and often what you see on the review you wrote yourself differs from what appears on the screen. The Q-tip test or swab test shows users, more or less, what the ink really looks like.