To an uninformed or stubborn engineer and product manager, “visual design” (that is to say, the graphic design of an interface with pixel-level precision and accuracy) is merely icing on the cake, fluffy and fun, for adding a certain “sparkle”. It’s not the deep, heavy, rigorous functionality that justifies the product’s existence, thus summoning the investments, valuations, and customer fees. 

The fact is, however, the clearly refined nature of a product’s visuals impacts several things that effect a product’s usefulness and thus the business’s financial value:

1. First impressions matter: Would you pay big sums for something that looks like a child finger painted? Of course not… Thanks to the iPhone and many strongly visual product experiences like Flipboard, Tumblr, Pinterest, Instagram, we are undoubtedly in a visual culture. People really do notice when something looks poorly crafted, signaling distrust, skepticism, and doubt. That first impression sets the tone of who you are and what you represent in the mind of the user and market. You rarely get a second chance to improve that first impression, just like in social situations in real life. 
2. Visual design is about communication: Forget the “prettiness” aspect for a moment. All those critical elements of the graphic design (hierarchy, balance, emphasis, contrast, whitespace, etc.) are choreographed not just to make something merely attractive, but also to convey the purpose of the product, the utility of the interface, including the content and functionality. If a button or drop down menu is lost in a sea of clutter, or obscured by overdone styling, users won’t get it.
3. Supporting eficiency and productivity: To help the product’s users get their jobs done swiftly with confidence and satisfaction, visual design is absolutely essential to that effect. Aligning elements properly so user can quickly scan, exposing the button clearly with strong labels so user can take correct action, cleanly exposed menu/navigation so user knows how to get back to where they were, or move around your product easily…All of this impacts productivity. The more clutter, chaos, confusion in the UI due to lack of properly applied visual principles (not to mention poor content strategy, another topic!), actually hurts user efficiency and productivity. The user is wasting precious seconds or minutes hunting around or doing trial-and-error to achieve a desired outcome. 
Let’s not ignore or belittle the value of visual design in designing amazing digital products. It could quite literally be to the business’s demise if left unattended or treated as an afterthought.