I do like Glyn Maxwell’s thoughts on this in his 2013 book, On Poetry (Oberon Books, 2012). I’m roughly quoting:

Poets work with two materials, one’s black and one’s white. Call them sound and silence, life and death, hot and cold, love and loss . .  don’t make the mistake of thinking the white sheet is nothing. It’s nothing for your novelist etc . . . for those folks it’s a tabula rasa, a giving surface. For a poet it’s half of everything. If you don’t know how to use it you are writing prose. If you write poems that you might call free and [Maxwell] might call unpatterned then skillful, intelligent use of the whiteness is all you’ve got. Put more practically, line-break is all you’ve got, and if you don’t master line break – the border between poetry and prose – then you don’t know there is a border. . .  a prose poem is prose done by a poet. . .

I like the physical sensation he creates here of the two spaces a poet works with – that almost dizzying cliff-edge of the line breaking, the powerful effect that must have, that it does have. And I’ve, personally, long puzzled over the prose poem, wondering ‘why?’. Some of my best friends write prose poems – look out Linda Black’s published by Shearsman – but they remain a closed book to me – ha ha. Why would you not exploit the white space Maxwell talks of? Surely not just because the art of the line break is a hard one to master? Oh – the discussions in workshops I’ve had! Briefly – something to do with breath, to do with reading the lines aloud, to do with the line having some weight before you snap it off. The rest – intuition.