I seem to recall a more practical reason for including the passage--as a
way to explain how Huck and Jim came to realize that they had gone past
Cairo.  In the deleted "Raftsmen's Passage," Huck relates:

"Ed said if you take the Mississippi on a rise when the Ohio is low,
you'll find a wide band of clear water all the way down the east side of
the Mississippi for a hundred mile or more, and the minute you get out a
quarter of a mile from shore and pass the line, it is all thick and
yaller the rest of the way across."

In Chapter 16 of Huckleberry Finn, Jim and Huck begin to suspect that
they had bypassed Cairo from a variety of clues--"no high ground about
Cairo, Jim said," for example.  Confirmation comes when Huck relates:

"When it was daylight, here was the clear Ohio water inshore, sure
enough, and outside was the old regular Muddy!  So it was all up with
Cairo."

In other words, the "Raftsmen's passage" is important as a source of
information for Huck and Jim to know when their journey is complete from
a change in the character of the water.  I recall having seen this
argument before, but I'm afraid I don't recollect where.

Good day to all!

Best regards,

Jim Edstrom