Drinking from the well is not one of the obligatory acts of the hajj, and a pilgrimage is complete without it. In practice almost nobody omits it, and the reason is that the water is the one part of Mecca a pilgrim can take away. The Kaaba cannot be brought home and the rites cannot be repeated elsewhere; the water can be carried, and it is.
That portability has shaped the whole social life of the substance. Returning pilgrims distribute it in small quantities to relatives, neighbours and people who could not travel, so a single journey supplies a much larger circle, and receiving it is a recognised way of sharing in a pilgrimage one did not make. It is given to the sick. It is used to moisten the lips of the dying. Some carry a portion specifically to be used in washing the body at their own death. In several regions it is touched to the lips of a newborn.
The scale of that demand has made the well an administrative problem as much as a devotional one. Millions of pilgrims arrive in a compressed season, all of them drinking and most of them carrying water away, and the modern arrangements for pumping, treating, testing and bottling it, and the regulation of how much a pilgrim may take out of the country, exist because of the volume. Export in unregulated quantities has been restricted at various times, and counterfeit water sold as Zamzam has been a recurring concern in importing countries.
None of this is peripheral to what the substance is. A holy thing that can be carried home becomes a holy thing that has to be managed, and Zamzam is the clearest case of that in Islamic practice.