Theory- Christmas trees, wreaths, boughs were originally brought into the house in winter (flu season) for the therapeutic (phytoncides) benefits.
As is often the case, what we are told is religious symbolism was actually homeopathic medicine.
This is why they denounce religion- further separate people from the traditions that foster good health.
No, I don't mean herbalism, or even herbalism if that's what you meant. Herbalism is herbs. Trees are not herbs and not all homeopathic medicine is micro-dosing, just the really toxic stuff is done that way.