The last line of the story zooms in on Elisenda, who is watching the old man disappear, flapping off into the horizon while she chops onions: "[. . .] she kept on watching until it was no longer possible for her to see him, because then he was no longer an annoyance in her life but an imaginary dot on the horizon of the sea" (13).
On the one hand, everything gets nicely resolved: the angel is gone, no murder necessary. But it also leaves us hanging. We never find out why the old man was there, or why he left; or whether he's an angel or just a winged Norwegian.
It's a little bit frustrating for those of us who like closure.
Even though the final line doesn't give us closure about the old man's identity, it might help us figure out what the overall message is behind this story. Elisenda hated and mistreated the old man while he lived in her house, even though she got rich off of him. But now that he's gone, now that he's back in the realm of the imaginary, she can go back to her ordinary life without him.
And this is probably the way a lot of us would deal with the sudden arrival of our imaginary sky-helpers: once they arrived—stinky, flea-bitten wings and all—we'd probably wish they were conveniently back in our imagination.