"Peter Clark" wrote

AIUI, it's because they're bladders and folds on the bottom could
gather water. If it was a wet wing they wouldn't need as many, like
the older models.

But they are wet wings, in question, AFAIK. Did you see the picture that
someone posted in a link? Each rib was forming a little damn that could
possibly trap a small amount of water, so a drain was installed to drain any
trapped water, in each bay.
--
Jim in NC