I don’t think the icon is used much (if at all) in the UI. And it is not used for the ‘favicon’ browser tab icon, which is often assumed. However, it is available to use in email templates by referencing the image URL in the template, for example:
Yes, you can also use external images for email templates.
I don’t think the icon is used anywhere in the UI. Other people have asked about this. The only information I could find said that it was used in a legacy certifications UI.