You know good design when you see it. You just don't have three hours to sit in Photoshop, or $500 to spend on a logo you might change anyway. The Designer makes the visual decisions so your work looks like it should.
You're launching a seasonal promo tomorrow morning. You tell the Designer the theme. Twenty minutes later you have a square for Instagram, a banner for your site, a thumbnail for the email, and a print-ready version — all in the same style, all ready to go. You pick your favorites and move on.