"Song of Myself" is considered a transcendentalist work because it deals with the idea that a spiritual state can only be achieved through one's self and one's own intuition and not through a set of learned beliefs like a religion for example.
I suppose people have to read it because it illustrates nicely one example of a form of poetry that was developed in 19th Century America that was markedly different from 19th Century English poetry. Transcendentalism was uniquely American and therefore it is good to know at least one or two examples of it and why and how it was different from say, Wordsworth or Keats in England who were considered Romantics.
Section Six deals with a child's question of "What is grass?" Whitman goes on to ponder the many different things that grass is or could mean metaphorically. Some critics have suggested that this is an analogy of the poem itself. From the Salem Literature site found on enotes, "The poem is for the reader what the grass is for the child: a “uniform hieroglyphic” for which there is no single Rosetta Stone by which it may be deciphered, other than the faith that Whitman cheerfully and insistently proclaims." If Section Six were seen to be different from the other sections, I suppose it would be in that it is the first place in the poem where the analogy of poem is to reader what grass is to child. This theme runs though the poem in various ways.