All images opened in Photoshop, regardless of their file format, are raster images (resolution dependent). Photoshop defines raster images as a grouping, or raster, of pixels that defines the detail of the viewable image. When you open an image in Photoshop, the resolution of the image controls how many pixels the image contains (its raster size) and therefore the amount of detail the image will contain when it is opened in Photoshop.
When you use Photoshop to adjust or resize an image, the resolution of the image gives Photoshop the information it needs to make the adjustments correctly. While it is not as easy as saying more resolution is better, photographic images scanned at higher resolutions will color correct, light balance, and resize better than images scanned at lower resolutions.