Why "VADAI" or "Vada"/"Bada" made of Black gram(Ulutham paruppu/Urad ke dal) has a hole in the middle of it?

nalla vegum. the hole gives it more exposed surface for the oil to fry it evenly. for paruppu vadai (we call it masala vadai), the ulunthu maavu is not as concentrated as it is in ulunthu vadai. it's tered by the paruppu. so it doesnt need a hole to cook evenly.