... how often has your grandmother said this right after you finished your bath?
The pottu (bindi) we keep on our foreheads is more than a custom. A spot of research (no pun intended) I did on this years ago, when Google was not so popular and I actually had to dig into books like Deivathin Kural and other books to find out, showed that the place between our eyebrows is an important chakra. Several important nerves converge there, and a mild massage at that spot twice a day helps activate positive vibes and energies and wipe off negative ones. Being a sensitive spot, only a mild massage is required, and the pressure exerted by our ring finger is just perfect for it. This is why elders always insist that we keep kumkum with our ring finger and not index finger or tall finger.
Although many of us have moved to sticker bindis these days, we should at least keep a small dot of pure turmeric kumkum along with it, because the mix of turmeric and lime helps to ward off infections.
I also remember reading that the spot between the eyebrows prevents somebody from fixing their gaze on ours and hypnotizing us. This was very important in olden times to protect women from black magicians who would lure them away, for sacrifices or other bad deeds. Even though that risk is not there today, it is still important for women to have a steady mind and not be carried away by the honey-coated words of untrustworthy people.
Perhaps if elders explained these scientific reasons to children at an impressionable age, they would not give up our customs so easily?