The ability to generate a chart in the spreadsheet off the data there, to then include in a word processor document, or a presentation. The ability to pull in data from a database like an address book. The ability to pull in media files organized in my photo application for a desktop publishing job. And so on.

There are plenty of good reasons to want integrations between the apps like Office, or iWork/iLife provide. And doing it by allowing each app to integrate with each other, instead of making one big monolithic apps seems decent to me.