My guess had been emphasis and artifical rhythm, so it's audiovisual: you'd read it out loud differently than continuous prose. Searching just now has sites saying it's about following the cadences of natural speech, which suggests it's only a visual form, at least in that aspect (free verse can use alliteration and rhyme and such, just isn't trying for a set pattern thereof.)

Some pages do talk about line breaks: https://kellyrfineman.livejournal.com/299689.html
http://mikesnider.org/formalblog/?p=654