For my purposes (embedded), UML statecharts are as good as any other way to do statecharts, and sequence diagrams are occasionally useful. The 10,000 foot view you started off with of big program blocks, also, especially to just draw out the processes in a system.
The complementary question, though, is how do you visualize data when coding? Some debuggers have features for visualizing data structures, time-series data, etc but there is plenty of room for improvement.
That's a good point that I omitted it. I was more thinking about things as the individual diagrams/views into the system (e.g. class diagrams, structure diagrams, sequence diagrams), and to me, UML seems more of a general superset collection of those individual views (and, importantly, agreement on what they are). While it contains many of the visualizations I mention, the visualizations themselves are more interesting to me than the general modeling language.
My list is definitely incomplete!