A successful last line is as necessary for a poem to work as a successful any other line, but some poems have one of those fabulously unexpected and (often) devastating/uplifting last lines. The kind of line that kind of ka-pows you even as it lands softly.
“Lying in a Hammock at William Duffy’s Farm in Pine Island, Minnesota” by James Wright and Rilke’s “Archaic Torso of Apollo” are the two classic examples.
“Photograph from September 11” by Wislawa Szymborska is another one of those poems with a perfect, an unexpected and exactly right, last line. And it’s one of the few really excellent 9/11 poems I’ve come across.