As we know water acts as a repellent to many tiny creatures that roam on the floor seen or unseen, hence the circle of water stops them to approach the plate when kept on the floor also they are attracted towards the part of the food kept outside the plate making it safe to consume the food. This practice is redundant while eating on dining table which is presumable cleaner than the floor. Those who still practice it on the table are just following the tradition without knowing the meaning behind it.