Why is Manolin necessary to the book The Old Man and the Sea?

In The Old Man and the Sea, the
character of Manolin provides an opportunity for dialogue and an opportunity for
characterization of Santiago to be revealed. 

Most notably,
conversation between the two reveals that Santiago is considered unlucky by the
villagers, but is reverred as a great fisherman by the boy.  We learn that Santiago has
gone many days without a fish--so many that Manolin is no longer allowed to go fishing
with him.  But we also learn that the boy would rather go with Santiago, and that
Santiago will not give up and stop going.  We also learn that Santiago is
undernourished, and possibly in danger of starving, at least
eventually.

Through Manolin the reader learns that Santiago
is a great fisherman, and though the boy's opinion is unsubstantiated when the words are
first spoken, they provide foreshadowing when the boy turns out to be
correct. 

In addition to dialogue and characterization,
Manolin's taking care of Santiago also creates pathos for the old man.  The reader sees
Santiago through the boy's eyes, and thus feels sympathetic toward him.  The reader
feels the same sympathy that Manolin does. 

Often, scenes
that feature a particular witness create more emotion than scenes that occur in relative
isolation.  When Manolin reacts as he does to Santiago's final victory/failure, the
reader's emotional reaction is heightened by Manolin's
reaction.