Reading Moby-Dick at 30,000 Feet
The poem is broken into three-line stanzas, except for the final one, which has four. Otherwise it's pretty freewheeling. All the lines hang around the same length, but there still doesn't seem to be any strict rule. Some lines and stanzas are end-stopped (meaning they have a comma or period at the end). But many of them end right smack dab the middle of a phrase and so roll right into the next line or stanza. In other words, this poem makes use of enjambment. A lot. Which forces us to constantly move through the poem, without a moment's pause, much as our speaker's mind makes leaps at a moment's notice. This looseness of form fits the way the poem moves rhetorically, what with all that moving from place to place and thought to thought.
So why does our speaker break up the stanzas at all? Or why isn't it more random? Well, one possibility is that a big block of text, or widely varied stanza lengths, would just make the poem more confusing and harder to follow. Since the poem moves around in time and imagination, we need our brain power focused on following that, and not some formal aspect of the poem. Having stanzas that are the same length establishes a pattern, so that it doesn't take up too much of our attention. Plus, those three-line stanzas are practically bite-sized, or thought-sized; they kind of match the attention span of our speaker.