I take this position for many fundamental reasons. Hackers scan for new IP addresses on a daily and nightly basis. Once they identify new ones, they scan for port weaknesses. If these services are left bound, individuals and small-office operators can suddenly find themselves sharing the contents of their hard disk or network with the rest of the Internet.
Personal firewalls protect ports from being displayed to Internet scanners.
This alone is a big boon to personal and corporate security and privacy. What's more, placing personal firewalls at desktops in larger corporations will help fight at least two problems. The first is Trojan horses. Some personal firewalls alerts desktop users that a certain program is attempting to make an external connection over a certain port to a distant IP address. A security-aware corporate user can permit or deny the connection based on knowledge of what that connection is attempting to do.