Most applications have configuration: how to open a connection to the database, what file to log to, the locations of key data files, etc.
Configuration is hard to express correctly. It’s dynamic because you don’t know the configuration at compile time–instead it comes from a file, the network, command arguments, etc. Config is almost always implicit, because it affects your functions without being passed in as an explicit parameter. Most languages address this in two ways:
Global variables are accessible in every scope, so they make great implicit parameters for functions.