Many porridge recipes require salt. Even if it will be a ''sweet'' porridge.
Is there any chemical reason for this ingredient?
No. It's just for taste. Moderate amounts of salt taste nice to people especially if we are accustomed to it in our food.
Salt tends to enhance the flavor of sweet foods as well. Recipes for cakes, cookies & other desserts often include a little pinch of salt, but you can always omit it if you would rather minimize your intake of sodium. From what I read, people on low sodium diets acclimate after a while to a lower saltiness level in their food, and don't even miss it.