The unfinished sentence in a poem may ask the reader to perform the ending privately.

A sentence left open hands the ending to the reader. The poem stops, and the completion happens privately, in words the writer never chose and could not have chosen. That silence is doing work, and it is work that only functions because it was left undone.

There is a version of this in conversation. Not every thought needs finishing aloud, and a question left slightly open often reaches somebody further than a fully argued case. Try saying less than you have prepared, once, and see what the other person puts into the gap. What they supply will fit them better than anything you could have provided.