It's more secure because you're minimizing the attack surface. The jailed process has no binaries an attacker can use to further exploit the system, not even a

It's more secure because you're minimizing the attack surface. The jailed process has no binaries an attacker can use to further exploit the system, not even a shell. It means that there's a minimal to non-existent /etc directory, meaning there is less information about the server and the network it's on. It also means there are fewer places to hide modules to persist, because you control so much of the tree. Going further, you can place the few executable files you need in a read-only filesystem, so attackers can't copy their own payloads in, further restricting what they can do. You can continue that process with firewall rules that are much more restrictive than what you could use with a more general purpose server, such as blocking any traffic that isn't to or from port 80/443. You can also virtualize the network of a jail, so an attacker wouldn't even get information about the network's layout from a compromised jail.