I found an article here that answers your question and added the quote I found and excerpted it:
"One note of practicality comes in the inclusion of a part that the original iMac lacked--a fan. The new machine runs a faster processor and is housed in a smaller enclosure. Thus, Apple had to include something to cool the air."
Here's the original article. There is no fan in the original iMac. It was very fascinating investigating this in the internet: