I personally use it for keeping track of important emails in our CRM.
Because this ID is a property of the email itself, and not something that this extension provides, it works everywhere. If your colleague got CC'd on an old email you'd like to point them to, you can pass them the message id and it will work for them as well.
It also works great in Inbox and the Inbox & Gmail Android apps.
See also the site at https://gmail-message-id-finder.co