BPM tips has a nice post with the opinions of several BPM and requirements experts covering the when and the how of using BPMN when collecting business requirements.?
In particular there are good comments from Joy Beatty of Seilevel, Jakob Freund of Camunda, and Bruce Silver of BPM Essentials - but many of the others were quite good as well.
I boil it down to this - if you're going to understand something, you should be able to draw a picture that represents it.? Any representation isn't 100% accurate, it is, rather, a model.? Models have a lot of utility in understanding the world by simplifying complex systems or ideas into simpler systems and ideas.? Separating the wheat from the chaff of understanding.
When modeling business processes, BPMN or a subset thereof is a great way to capture the basic model, which can be expanded to an executable model by tools that support an execution engine.? (For example, you can see the migration from modeling to execution in Blueworks Live and IBM BPM in the IBM family of products).