Not just a popular tool for photographers, Photoshop is a popular choice for digital artists of various mediums. This sets the two platforms apart, with Lightroom being developed primarily for Photographers.
For digital artists of all mediums, including photography, Layers are an essential tool for achieving the finest level of control over image manipulation, with many layers often added and masks applied to a very specific area of an image. Layers are present in Photoshop, but not in Lightroom.
The ability for pixel-level control over images and to work with multiple images to create composites is one in which Photoshop has alone, in terms of the two platforms which we are comparing.