In Hindu culture, the Tulsi plant is regarded as a holy plant. In large houses, it is always placed in the center of the courtyard. The women of the house worship it every morning and light a lamp for it every evening. According to some beliefs Tulsi is considered to be a manifestation of Goddess Lakshmi and worshipping the Tulsi is known to ward off evil and bring peace and prosperity.
If you think about it on a scientific basis then Tulsi leaves are supposed to have many medicinal properties. They can ward off cold, asthma and are also known to improve immune function. It used to be a common custom to chew some leaves of the Tulsi everyday to keep oneself healthy.
It is interesting how many Indian customs and traditions not only have spiritual significance but also a scientific basis.