Mail filters have a number of uses. The most important are spam filtering, dispatching to multiple mailboxes by topic or sender, and auto-answering mail. Typically, you set up mail filtering by putting a program alias for the filter program in your .forward file, and writing a file of filtering rules. The format and location of the filter rules file varies between filter programs. As these names imply, it is an obnoxious form of advertising that stuffs your mailbox with form letters.
Spam tends to be both verbose and illiterate.
It's a waste of time and a huge waste of network bandwidth. The spam epidemic seems to have peaked in mid-1997 and been slowly in decline since, but it can still be a serious annoyance. If you're being deluged with spam, get educated.