Because we had to come to this earth, plants came into existence first so that we could come.
In our country anything that is beneficial to life and which enhances the prana-shakti has been considered sacred. Tulsi as well as lemon has been considered very sacred. Neem tree also is supposed to be very sacred. To get rid of any infections neem and lemon are very valuable.
Devi (Goddess) is believed to reside in all these.
Tulsi is good for the chest – the residence of Vishnu. The chest is for protection and preservation, and that is why they say Vishnu and tulsi are married. Meaning these two have a very close connection. If you have a cough, you should drink a brew of tulsi.
In the same way Bilwa leaves are very good for the mind and the nervous system; it has a calming effect. Also if you are having stomach trouble, Bilwa leaves are recommended.
So whatever we consider sacred is found to be useful for various parts of the body.