Hemingway uses a technique called "stream of consiousness" in the final italicized paragraphs of "The Snows of Kilimanjaro. Why?
Stream of conciousness is the literary way of expressing thoughts as they occur. It may appear disjointed at times or non-sensical to anyone of than the character who is thinking or feeling those thoughts.
It has always been my opinion that the reason for Hemingway's switch to stream of consciousness is because Harry is dying. It realy gets inside his head and sees his fear of dying, whereas before, when speaking to his wife, he puts up a brave front. It was rare for Hemingway to use this type of style, as usually his writing is short, dry sentences. It also serves to highlight the real world (where Harry is dying from gangrene) to his dreams of Kilamanjaro.