ISA Server 2004 fills many roles at many organizations. In certain environments, it provides dedicated web proxy capabilities. In other locations, it serves as a dedicated OWA reverse-proxy server. All these deployment scenarios utilize specific pieces of ISA functionality, but the full range of ISA functionality can only be had when it is deployed as a dedicated Application-layer firewall.
The capability of ISA firewalls to provide for robust and secure stateful inspection of all traffic passing through them gives them an added edge over traditional packet-filtering firewalls. In addition, the capability to provide for advanced logging, server publishing, and VPN functionality positions ISA squarely in many environments for the long term.