Your mental model of designers seems to be "people that make stuff pretty". In the field of software development that's typically not even the most important part of a designer's job. The post specifically mentions HCI. They typically help you A/B test you stuff and are very data oriented and use quantitative methods. The user centered design process (one example being ISO 9241-210) focuses on the user and users' needs. They typically can crank out prototypes (that you can interact with) at a great pace which helps a ton with testing all sorts of hypotheses about your product.

That seems really valuable. Especially since they usually can make stuff pretty as well ;)