They also have good design sense (UX). So they know what makes an intuitive UI and where to put things. They also can fill in on the designer's blind spots when doing the implementation of a design.
In bigger projects and companies, there might be a dedicated person for each aspect of the process, a designer, and front end developer, etc. But the responsibilities are also different in that case. For example, a UX person doesn't just tell you that something isn't as intuitive as it could be or even improve it, but conduct actual research like A/B testing etc. to properly asses the usability, compare different design solutions and so on.