Class diagrams, the most used of UML diagram types, are not exactly for system design, they're too low-level, and yes, "Round-trip engineering" that generates code from class diagrams (and class diagrams from code) was advertised as important feature of advanced UML tools.
Converting classes into UML diagrams can be usefull regardless if you classify UML as obsolete or not. It's a nice why to understand how a given class or interface is used throughout.
