So what's the big deal? Especially if you are an Illustrator user, you may be wondering why you would choose to design documents using InDesign rather than the program you're most comfortable in. And the answer, for certain documents, might be that you wouldn't.
InDesign's strength is definitely in the category of page layout. Although its graphic tools are similar if a little less robust than Illustrator's, the biggest plus is its ability to handle larger documents. Illustrator is still, at its heart, a graphics program. It might be the perfect way to create a one-off document, like a poster or labelafter all, you can still draw vector objects, create and edit text, and import images. But if you need a resource for magazines, newsletters, or more involved documents, it's easy for Illustrator users to move straight into InDesign.
Because it serves as more of a production workhorse, you might find that InDesign has some interface features that are different from those in Illustrator. Palettes can dock along the side of the InDesign workspace, giving you more room to maneuver, although most of the palette options are the same. You will find that more flexibility with text exists in InDesign, and more complicated design effects can be created in Illustrator.
All in all, although there is some overlap between Illustrator and InDesign, just as there is with the other programs in the Suite, both have a place for the designer and can peacefully coexist on the same hard drive while you take advantage of their unique strengths.