For what purpose does Jack use the chant and the dance?

Psychologically speaking it is highly unlikely a child of
his age had any "planned" reason for using the chant, and dance.  It is more of a primal
instinct than it is a choice "to do or not to do."

Such
social rituals have existed in human culture for thousands of years.  They create group
identity.  Rituals have a great deal of symbolic value to humans.  They feed spiritual
or emotional needs, strengthen social bonds, demonstrate respect or submission, state
one's affiliation, earn social acceptance or approval, or sometimes just for the mere
pleasure of it!

In the Lord of the Flies that chant and
dance would have filled the boys with a sense of belonging.  Anyone who chose not to
partake in such activities would be considered an "outsider" to their group, and often
outliers are viewed as threats to the social cohesion of the group.  When social groups
feel threatened it can get really violent as is demonstrated in the book.  All religious
wars have been fought because of the "threat of the outsider."