As we all know, in Hindu Temples, after the Mangalarti has been done, priest will start serving the theertha which is a Tulsi water. Why is it served in all the temples? Also sometimes they serve milk and fruit mixtures.
It is not used in ganesha temple, since ganesha and tulsi have a bad history. But It is used with every pooja to other gods like without tulsi leaves no prasad(holy food sacrifice) is complete, and the water that is kept in front of god in temple and is seen as their drinking water.It is distributed as prasada like holy water to every person after aarti.
And yes there is a scientific and medical reason for this , tulsi provide relif from congestion, mixing it in water and drinking it balance kapha in body, which is very crucial for curing many diseases and it is also said that kahpha (cough) blocks prana channels and causes different problems.More info