Google added to the Help documentation for the logo scheme / structured data that you can now use with the ImageObject type to specify a company logo. This is in addition to the URL type.
According to Google, this offers more flexibility for this scheme.
So far, only the URL type for the logo scheme has been specified in the help documents.
Why should an ImageObject type be used instead of the URL type? If you want to provide data about the image, you must use ImageObject because you cannot add properties to a URL value. Such data can be the height and width of the image or its author or caption, etc.
You can find more about this ImageObject at Schema.org.
Forum discussion at Twitter.