Since the flavors and aromas of spices and chiles are volatile, they can vanish from preground versions long before you buy the jar. Toasting and grinding whole spices releases these volatile compounds, along with their essential oils, exactly when you need them for a dish, not before. Plus, toasting triggers browning and caramelization of the spices’ proteins and sugars for even more flavor.
Whereas blooming spices involves cooking ground spices in fat to help reawaken their flavors, with this technique spices are toasted whole in a dry pan, without any fat. Since it’s often the first step in a recipe, the toasting can be done in a Dutch oven or whatever vessel you’re already using rather than pulling out a separate skillet.