I believe the idea is (or was) that the e-mail app will use a "content provider" component[1] to give other apps access to data as appropriate.
[1] https://developer.android.com/guide/topics/providers/content...
The point of this structured data format is that you don't need to give the calendar app fill access to the email; just to the attached calendar info included in the email, which can be attached to the intent.
To do this the calendar would need to access the emails - which would need either IMAP built in to the calendar app [EDIT: or to change the android permission model].
Edited - finished sentence