Fasting niyat(dua)

Asslam o alikum. In Islam the new day begins after maghrib , right. When we do sehri and close our fast at fagr , we do the intention(niyat) of fast. From fagr to maghrib(when we open the fast) it is the same day. But the intention(niyat) of fast is: I intend to keep the fast for tomorrow in the month of Ramadan.  What is the reason behind this? Thank you.