Sequence diagrams are probably the most useful UML diagram. I don't know of a simpler way to specify and document workflows, no matter the scale (from function to whole system)
Now that I think about it, sequence and state diagrams are pretty much the only programming-related diagrams I see used in the documentation at my telecom job. I even see sequence diagrams written in code comments from time to time, they're always appreciated.
- Sequence Diagram: great for understanding distributed workflows
- State Diagram: state machines are wonderful tools and having a standard for representing them is a win, IMHO
- Parametric Diagram[0] (SysML): feels like Class Diagrams for functional programming
[0] https://sysml.org/sysml-faq/what-is-parametric-diagram.html