It was common for both men and women to wear this tilak/ dot in ancient India. Currently this has fallen out of fashion to some extent and men dont wear it a lot.
To understand the reason for this tilak, we first need to understand that the Hindu culture has through ages combined science and tradition together, so that good scientific habits could be carried down generations in the form of tradition.
If you search for kundalini meditation online you will find that the Agna Chakra which is a very important chakra in meditation is located at the centre of the forehead where hindu's put a red dot.
The Atma or the soul of a person is also located in this position according to Hindu scriptures.
There is also a meditation called third eye meditation or Jangama Dhyanam which focuses on concentrating on this same spot which is in the centre of the forehead just above the eyebrows.
So the tilak or the red dot at the center of the forehead is exactly applied at this point. There are various theories like
1. The red dot reminds us who we really are not the physical body but the soul or the Atma that is located at the center of the forehead.
2. This red dot is to stop the flow of energy outward from the Agna Chakra.
3. This red dot protects us form unwanted direct influence. Its said to act as a shield to our mind from unwanted influence.
I believe all these can be true.