Code service account is a function provider. By connecting to a provider, you can use its functions from CodeMash. By default, when you enable code service, default CodeMash provider is added which allows you to use CodeMash implemented functions or create your own custom functions.
You might want to add your own provider account with your own functions. This has its own benefits such as an easier development environment (provided web code editors or integrated deployment options). The following are the supported function providers which you can use from CodeMash.
Google Cloud Functions
How to setup each of them, follow the links below.