Charring/roasting ingredients is a way to concentrate and deepen their flavor. It also somehow gives the dishes where they are being used an interesting and rustic feel.
Charring tomatoes, aside from concentrating and deepening their flavor, brings out their sweetness and juices.
To char or roast tomatoes, you can either place them directly on a grill, or on an already hot comal or griddle over medium heat, for about 10 minutes, turning them around a couple times. However, for me the easiest method is to place them in a single layer in a baking sheet or shallow pan under the broiler for 7 to 10 minutes. Turn them once in the process.
You know tomatoes are ready once they are completely cooked through and mushy, their skin is charred, blistered and wrinkled, and juices have started to come out.
Place unpeeled and pricked cloves (so they don't make popping sounds) on the already hot comal over medium heat. Turn form time to time until it is charred on the outside and soft but not burnt on the inside, about 6 minutes. You can also place under the broiler for 3 to 4 minutes.