1: Consistency of color across the design: Using the same swatch throughout the design process assures that all element are colored correctly.
2: Easy buildout of adjacent designs: The use of a spot color in a series or family of designs can easily be used as the “Flavor Color” in a family of products reducing the complexity of the printing.
3: Small Text Knockouts: Using a spot color instead of a four color process build allows you to knockout small text to the paper color in designs that would other wise fill in and be unreadable.
4: Can be used as a Technical Ink: If dark enough, the ink can be used for technical purposes like Bar Codes, Lot numbers, Nutrition Facts, and other legal text. Your printer will love you for giving them this as an option.
5: Closer Adherence to Design Intent: If you create a package design for a major customer out of nothing but four color process builds (something that ends up on a store shelf), the first thing they will do is give it to a guy like me to simplify it and make it more easily reproduceable. It will be considered amateurish (because it is), and you now have a credibility problem.