I think I understand now. It had the same effect as importing into a
separate calendar within Outlook. And I was wrong on the import options
-- they explicitly list .ics files.

MikeH wrote:
I wanted to move my calendar from Mozilla Sunbird to Outlook 2007. The
import options for Outlook didn't look too helpful so I first started by
exporting my Sunbird calendar to an .ics file on my desktop. For grins
I clicked on the file and it opened up in Outlook 2007 -- alongside its
still empty native calendar. I shunted the empty calendar off the
screen so now I have my original calendar neatly displayed in Outlook. I
haven't found in the help areas just what this all implies. Am I done?
Will the calendar now function like the native calendar? I do not need
to share appointments if that is an issue.