Can you describe the process of fermenting dosa or idli batter, and why it's done?

Dosa and idli batter are made by soaking and fermenting rice and lentils (usually urad dal) before grinding them into a smooth paste. The batter is then allowed to ferment for several hours or overnight, during which time natural yeasts and bacteria break down the starches and sugars in the mixture, resulting in a light and airy texture. Fermentation also enhances the flavor and digestibility of the batter, making it easier to digest and more nutritious. Once fermented, the batter is used to make dosas (thin, crispy crepes) and idlis (soft, steamed rice cakes), which are popular breakfast items in South India.