I think "preview" is meaningful for Safari because their TP releases have no version number, but what does it mean for Firefox and Chrome that give version numbers for nightly/canary builds?

In other words, in what situation would "preview" be preferred over actual version number?

"chrome": {
  "version_added": "preview", // why not an actual version number?
  "flags": [
    {
      "type": "preference",
      "name": "enable-experimental-web-platform-features",
      "value_to_set": "enabled"
    }
  ]
},