Chargers Serve as Place Mats
Chargers, large decorative plates, are placed on the table before the meal begins. The soup and salad courses are served on top of the charger, which is usually removed before the main course, although that is not required. In a sense, when placed on top of the tablecloth, the charger itself serves as a place mat for the first two courses, making an actual place mat unnecessary.
Place Mats Add Additonal Color and Texture
If the chargers coordinate with the place mats in terms of color and scale, they can add layers of color and texture and make your table visually appealing. Using a place mat also protects your table or tablecloth if you remove your charger before the main course is served.
In most cases, using a place mat under a charger is not necessary. Placing on top of a tablecloth is fine. Chargers are decorative elements that protect the tablecloth or table and add color and formality, essentially serving the same purpose as place mats. If you are concerned about your tablecloth getting dirty or your table being damaged, it is acceptable to leave the chargers on the table throughout the meal.