This allows the onion layers to swell and let any grit rinse out. You don't want anything like that in a great soup.
and toss them on the compost pail and continue chopping and rinsing. You will always find more dirt and grit in this area because of the way leeks grow in soil. I goo almost to the tips of the greens, especially if they are still bright and tender.
Here you can see the ratio of leeks to potato. Obviously, that can be reversed if you so choose. Me, I like the onion.