I think the main skill that designers bring to the table is empathy. Unlike traditional artists, who can safely ignore what other people think, designers have to be interested in how something feels to their audience. This is important if you're building things for people who aren't like you and don't think like you, and you want to get their attention, get them to think what they're buying is useful and usable and yes, maybe beautiful if that matters, and the people who made it are trustworthy and reliable.
Design is about communication--it's a kind of language. Who could argue that words don't matter, only function matters?