During the story it is made obvious that the couple's relationship is going sour. Hemingway uses the cat stuck in the rain, with nobody to care for, to symbolize the wife who longs to be loved."
"The story presents the American couple as emotionally barren, isolated by their own self-absorption. The husband is unconcerned with his wife's malaise, never rising from his supine position on the bed, and even growing angry when she attempts to express her desires. The wife is nameless; shunned by her husband and lacking a sense of self-hood, she is like the 'poor kitty out in the rain.'"
I believe I also saw a reference to the war monument being present in order to set a more somber mood to the story.
Here's tons of reference material (enough to make your head hurt):