Two replies of mine in a conversation today about the purpose of formality in poems. Comments welcome!

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You say that division into lines is something free verse shares with all other kinds of poetry. This intrigues me: from my point of view, "line" is part of metre, and as such is structural and should be more or less invisible. For example, in the first two lines of this sonnet of Cope's, which I chose because it's heavily enjambed so the line break has no connection even to grammar:

Not only marble, but the plastic toys
From cornflake packets will outlive this rhyme

the line break means nothing more than that we've had five iambs, and in reading the poem aloud (at least, if I was reading it) it wouldn't have any representation at all, no sound and no pause. But I've never understood quite what it is that free verse uses line breaks for.

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As to the purpose of form: poetry is first of all a spoken medium. Metre and rhyme are primarily a mnemonic device to assist this; secondarily, they become a skeleton to build the poem around.

The biggest danger specific to formal verse is letting the form drive the poem-- or as you put it, turning it into a crossword puzzle. It's a beginner's error, though some people never get past it. Formal verse done well should hide its structure, not shove it in your face like Brutalist architecture. It should play with imagery and wordplay and emotion every bit as much as any other kind of verse. Sometimes a reader tells me they came back to a poem of mine after a few days and realised it was formal. And I take that as meaning I've succeeded.