Any system is can be broken down into smaller chunks, and these in turn broken into yet smaller chunks. However, unless your chunking is done by using some organised theme, there is a risk of ending with lots of random pieces that do not connect well to each other. Business events are the best way to decompose systems so that the resultant pieces are coherent, meaningful, and suitable for implementation cycles.
Business events are highly effective for agile teams. A business event (or correctly, the response to the business event), is a stand-alone chunk of the problem with a well-defined outcome. It also has well-defined input and outputs. This makes it not only a convenient unit to study and find its requirements, but also a sensible unit to develop.
This article discusses how business events and agile development are a potent combination.