CentOS is a very popular Enterprise-class Linux Distribution derived from sources freely provided to the public by a well known North American Enterprise Linux vendor. CentOS continues to gain popularity as Windows administrators move to Linux to save money and for the performance gains.
The Haraka mail server is able to used common plugins like Spamassassin to enhance the features that it provides. Using Spamassassin with Haraka is easy to do and reduces the normal resources used by a mail server.
The primary reason for adding these repositories is that neither the CentOS repository nor the rpmforge repository contain the same packages that were available in CentOS 5.x. There is some serious lag time which will cripple your ability to move to CentOS 6.