Because both Photoshop and Illustrator have layers, it only makes sense to want to use layers from one product in the other one. It's nice to be able to manipulate individual layers from a Photoshop image in Illustrator, so you can make adjustments on a portion of an image. Before you embark on this, however, keep in mind that a few things in Photoshop won't work properly when you open a Photoshop layered file in Illustrator, including Layer Effects, Adjustment Layers, and Blend Modes. If your Photoshop file has these, you'll want to either flatten the image in Photoshop—or at least those effects—before bringing the file into Illustrator.