The Zoroastrians or popularly known as the Parsis, are people of a unique religion that has been here for a long time and have survived significant threats. And in the process they have also evolved yet kept many of their heritage and much of their culture alive.
Death is the one ultimate truth of life and has been viewed differently by every religion but respected for its unavoidability. With death, we leave behind the material wealth of our bodies and it has to be returned to nature. It is where the Zoroastrians carry their unique ritual and in turn face their unique problems.
These towers are analogous to the burial grounds or cremation sites of different cultures.
Yes, this is how the Zoroastrians have been disposing off the bodies initial ADs i.e. 6-7AD and that's the documented proof, the practice is far more ancient.
The bodies were believed to possess evil as soon as the soul left the body. Also their belief of the sanctity of earth and fire, they were to be kept away from both until they decomposed. The bones were later buried. This was also the final act of charity from the dead.
These towers were considered sacred sites and thus developed and maintained by religious authorities and authoritative personnel. They also had powers to restrict use of the site.
Although many have resorted to burial due to the closing of these sites as most of them fell under the city perimeters and between general populace.
With the extensive use of chemicals, pollutants, and other activities, the population of scavengers have tremendously decreased in last century. The process which took less than a day now takes more than months and has thus directly affected the culture of Zoroastrians.