As we know, JSON does not allow comments, and only root level comments are valid in a template.
If there was a Description key, that doesn't do much beyond validating correctly, it would be a fantastic place to comment each step of the template. This becomes especially useful if you have a lot of provisioning steps, or have steps for workarounds, or steps of non-obvious intent.
Example from the tutorial
{
"variables": ["..."],
"builders": ["..."],
"provisioners": [{
"description": "Install latest Redis version; WARNING: This may conflict with foo",
"type": "shell",
"inline": [
"sleep 30",
"sudo apt-get update",
"sudo apt-get install -y redis-server"
]
}]
}