From an Apple support discussion on why sharing documents online via iWork.com is better than emailing them as attachments:

I don’t need to send to 20 other people, then get 20 mails back and read through those 20 mails. At the same time, each of those 20 has no idea what the other 19 are thinking and thus means there will be many overlapped ideas in those 20 mails I have to go through.

That nicely sums it up.