Theertham is the holy water offered to god, which in turn is shared to the followers of god. The water is not only holy, but there are other healthy reasons for anybody to drink the same water.
The holy water is made of water, Sandal, Holy Basil, Camphor mainly. Some other temples may also add other herbal items. This water that is mixed with these various herbal items, has the power to kill germs and bacteria, and prevent you from getting any basic health issues, like common cold.
So, if you in take some Theertham after visiting the holy place, it not only gives you a very good feeling in your mind but also, keeps your body healthy also.
That is why, it is good to consume a little Theertham from any temple.