Anti-spam appliances are software or hardware devices integrated with on-board software that implement spam filtering and/or anti-spam for instant messaging (also called "spim") and are deployed at the gateway or in front of the mail server. They are normally driven by an operating system optimized for spam filtering. They are generally used in larger networks such as companies and corporations, ISPs, universities, etc.
Reasons hardware anti-spam appliances might be selected instead of software could include:
References
Anti-spam appliances Wikipedia(Text) CC BY-SA