When Plan objects are created there is no way of adding attachments right from your desktop, instead at a minimum you need a deliverable that can be linked to a folder document in that project.
Giving ability to add attachments gives additional flexibility and ease of locating related objects. This is consistent for several objects in Windchill that have ability to add attachments. (ex: change objects)
in addition there is less picks for users and additional capability of also attaching documents to Activities.