[ZeppelinDePlomo]

You don't "design" diagrams, they are design tools. The point is communicating and formalizing models to iterate ideas more quickly and cheaper than coding or prototyping (of course, they work up to a point). They are ways of visualizing interactions, states, operations and so on, the "heart", the main "idea" or "the point" of a system in a way that implementation details don't get in the way of the main innovation and/or core value added.

It's a really useful skill.

...but "UML" as a brand to add to your resumé is pretty much useless.