1) Proxies can hide the internal address structure of your network.
2) Proxies can better log the nature of traffic crossing the firewall
than packet filters
3) Packet filters are generally (A lot of people may disagree with this)
faster than a proxy server.
4) Packet filters are generally easier to secure than a proxy server.
(In terms of fewer vulnerabilities to be protected/removed)
None of these observations is an absolute. It all depends on the technology
employed, and how much time and money you have available to do the job.