What purpose? Writing config files? Lua is a programming language (a nice one too). It's code. Code should not be used for config files, nor for data serialization because once you eval it you are executing it.
Yes, Lua was originally written as a language for rich configuration files and has grown out of that into a more fully featured language.
It's also one of the easier languages to sandbox, since you can evaluate user provided code in a custom environment that only contains the functions you deem safe. You can even use the standard debug hooks to set an upper limit on the number of instructions a script can execute to prevent someone from creating an infinite loop in a config file and locking whatever thread is reading the config.
It's not very appropriate as a data serialization format, or as machine written config, but the parent post specifically asked about human written configuration files.
It's also one of the easier languages to sandbox, since you can evaluate user provided code in a custom environment that only contains the functions you deem safe. You can even use the standard debug hooks to set an upper limit on the number of instructions a script can execute to prevent someone from creating an infinite loop in a config file and locking whatever thread is reading the config.
It's not very appropriate as a data serialization format, or as machine written config, but the parent post specifically asked about human written configuration files.