It has been a Hindu custom to wear the vermilion on the forehead at the hair parting. There is no actual science behind it.
However, some theories state that the partition of hair on the forehead is believed to have the pressure points for the sexual desire. Applying vermilion at this position activates the sexual desire and that is the reason why only married women are allowed to apply it. There is no practical confirmation for this theory.
The sindoor is prepared by mixing turmeric, lime and mercury. Mercury is believed to control blood pressure and activates sexual desire. The position where hair parts ( if we comb the hair in the traditional way) is the place where pituitary gland is believed to be located. It is the pituitary gland that is the center of all feelings. So, that must have been the reason for the tradition.
However please note that there are no scientific authentication available for the tradition.