I asked a similar question on this forum not long ago. Having enjoyed the benefits of a Caddy 2 server in front of Phoenix, I would now go as far as saying that it was a no-brainer. Here’s an updated list of the things Caddy 2 helped me achieve (I presume that most would likewise apply to HAProxy):
• adding support for the latest protocols, such as TLS 1.3 or HTTP/3 (experimental)
• serving a maintenance page when the Cowboy / Phoenix web server is down
• adding security and other headers to all requests, or to a broad range of requests
• adding an authentication token header, e.g. to protect a staging server
• redirecting automatically from the domain root to the www subdomain
• dealing with the issue of trailing slashes in URL paths
• hosting static websites on the reverse proxy server itself
• serving multiple domains / subdomains that share a single IP address
