One thing to understand is the relationship between the raw coffee bean and the roasting process. Compare it for a moment to your breakfast toast using two different breads. If both breads are both cooked to a low roast level, the flavours of the bread, will shine through. If however, both breads are instead toasted/roasted to a much darker level, the flavours of smoke and the toasting process is the dominant flavour with both breads tasting more like 'well cooked toast', than 'toasted bread'.
The same thing can be said for BBQ food. and the same is true of coffee beans.
Caffeine is reduced during the roasting process as it accelerates chemical interactions which also breaks down caffeine compounds.
Dark roasts are also known as French Roast or Espresso Roast, which relates to the roasting level, rather than the bean which can be the same.