A common view is that Linux is not vulnerable to viruses, so why install an antivirus solution? While it is true that there are very few viruses for Linux, the primary objective is not to protect the mail server from infection, but to reduce or eliminate any risk to recipients. Your organization may have client PCs running Windows that are susceptible to viruses, or you could receive a virus laden e-mail that you may forward to a customer or business partner.
One of the many options for filtering with Procmail is to remove executable attachments from e-mails in order to protect your system from possible virus attacks. This will be, at best, a crude operation; at worst, it will remove files that do not contain viruses and possibly leave other infected documents such as scripts that are not executables.
It is also possible to scan e-mails on the client side. But in a company environment, it is not always possible to rely on every individual having their machines...