Prose usually lets the sentence set the pace. In poetry, the line can interrupt that pace. A word at the end of a line hangs in the air for a moment before the next line changes it. Poems also use pattern, silence, and sound in ways prose can use but does not usually foreground.
The border is porous: a prose poem can have no line breaks and still make us read for rhythm and image. The useful question is what the writer asks us to notice.