Usually, you add salt to water in order to boil the water to cook daal . Adding salt to water adds flavor to the water, which is absorbed by the food. Salt enhances the ability of chemoreceptors in the tongue to detect molecules that are perceived through the sense of taste.
Another reason salt is added to water is because it increases the boiling point of the water, meaning your water will have a higher temperature when you add the daal, so it will cook better.