It's all about the caramelization, which happens during roasting as the sugar in the vegetables (yep ... vegetables DO have sugars) are heated to a point where their colors and flavors are changed (deliciously). As Bon Appétit so perfectly explains, "When sugars caramelize, they develop nuttiness, bitterness, toastiness, and even a little bit of buttery creaminess." Yeah ... YUM!