Final answer:
Monopolistically competitive firms advertise to increase demand and create inelasticity in their demand curve, enabling them to sell more products or charge higher prices to boost profits. They can also use other strategies such as product enhancement and promotions to increase demand.
Step-by-step explanation:
Monopolistically competitive firms advertise in order to increase demand for their products and to make their demand curve more inelastic, meaning consumers are less sensitive to price changes. By differentiating their products and creating brand loyalty through advertising, firms can potentially sell a greater quantity or charge a higher price, thus increasing their profits. Additionally, aside from advertising, these firms can increase demand for their products by improving product quality, offering exceptional customer service, engaging in promotional offers or sales, enhancing product features, or through innovation.