A good idea can only go so far without a great pitch. A cautious client will never make a bold leap unless they’re sold on it. And a beautiful design can be quickly dragged down by poor assets.
High quality, timeless mockups are hard to come by. They are either branded and quickly dated (I'm always looking back on my portfolio and cringing at the old school device mockups), or designed at a strange angle, or hard to adapt, or so low quality they cheapen our work.