In poetry, what's the function of a colon?

They can be used by certain poets to prepare the reader for an exponential magnification of emphasis on what is about to follow. A good example would be the second and third stanzas of the poem I Wandered Lonely as a Cloud, by William Wordswoth.

I wandered lonely as a cloud

That floats on high o'er vales and hills,

When all at once I saw a crowd,

A host, of golden daffodils;

Beside the lake, beneath the trees,

Fluttering and dancing in the breeze.

Continuous as the stars that shine

And twinkle on the milky way,

They stretched in never-ending line

Along the margin of a bay:

Ten thousand saw I at a glance,

Tossing their heads in sprightly dance.

- (notice how the colon has prepared the reader to the explosion of sight and color of ten thousand flowers!)

The waves beside them danced; but they

Out-did the sparkling waves in glee:

A poet could not but be gay,

In such a jocund company:

I gazed-and gazed-but little thought

What wealth the show to me had brought:

- (Notice how the first colon underscores the exceptional intensity of the narrator's gaze that follows it. Then comes the second colon that is meant to underscore the strength of the memory of such vision still powerful during the narrator's reminiscences).

For oft, when on my couch I lie

In vacant or in pensive mood,

They flash upon that inward eye

Which is the bliss of solitude;

And then my heart with pleasure fills,

And dances with the daffodils.