What's the use of challenge password in build-key-server and build-key from Easy-RSA?

All the OpenVPN / Easy RSA tutorials that I've found, advise to setting an empty challenge password while building the key for the openvpn server.

Anybody knows why? What's the intended use for the challenge password in Easy RSA server's keys?

And what about client's keys? I see that a build-key-pass esists to generate encrypted client keys, but no server equivalent esists. Still, both build-key and build-key-pass ask for a challenge password.