Why is it important that an IaC language is stored as plain text files?

Plain text can be managed in a version-control system like Git: changes are checked in, the history is visible, and you can go back to an earlier configuration.

Once infrastructure is described in text files, all the tools software developers use for source code apply:

• Managed as text files: diff them, search them, edit them with any editor.
• Changes are checked in (committed), each with an author and a message.
• History of changes: you can answer "who changed the firewall rule, when, and why?"
• Return to earlier configurations: if a change breaks something, revert the commit and re-apply.

A binary format or a GUI-only configuration gives you none of this. That is why IaC tools deliberately use text formats like YAML, JSON or HCL.

Go deeper:

• About Version Control (Pro Git) — what version control gives you: history, comparing and restoring earlier versions.
• Infrastructure As Code (Martin Fowler) — "version all the things" and keeping infrastructure definitions in source control.