Using a reverse proxy can thwart some exploits targeting the webserver.
- Reverse proxies are usually simpler than webservers, so they are (in theory!) less likely to have vulnerabilities.
- If the proxy is exploited, this does not yet give the attacker access to any data on the webserver or other systems behind the DMZ
Note that this only applies to low-level exploits targeting the webserver directly. Any application-level exploits won't be affected by this at all. The proxy might also directly forward some exploitative connections as they are, so they work even through the proxy.