At the risk of raising the ire of those that want to avoid precise meaning in their process flow diagrams, I recommend a study of Anatoly Belychook's handy primer on the use of the signal event.
I would emphasise that the signal event allows the ultimate "late binding". You do not have to design the world process ... only the bit you are in control of at the moment. When you come to an event that you know will be useful to another bit of the enterprise ... Signal ... then when a use is found in another bit of process design you do not have to redo the process design work again. Of course, you will need a way to hunt down these useful signals and some standardisation of how they will look in your enterprise.
For those of us that look forward to the executable process, Signals also provide a simple communication method between parallel flows within the same pool and avoid the process designer having to think too much about implementation level stuff like writing data to a store somewhere.