How does it save water by using a reusable water bottle?

Answer 1

Using a glass or earthenware or metallic water bottle saves on water because it uses less water to fabricate a lifelong water bottle than what it takes to fabricate hundred of plastic water bottles.

It also saves on water because you don't have to import oil from the other side of the world, build a supertanker, etc., although this would probably be more relevant to environmental concerns in general rather than water in particular, and glass or metallic bottle factories use imported materials as well.

A reusable hard plastic water bottle would save on water as well, but it usually contains BPA or other toxic compounds to make the plastic hard and resistant instead of naturally soft.

Hard or soft plastic water bottles of any form are an environmental aberration except for extreme sports. You can survive a few hours until you reach a source of (hopefully filtered) tap water. You should use a metallic bottle for biking and hiking, but you don't need one at school, on the playground, at work or in the car.

At home, use clay jugs. They provide naturally cool water (because the evaporation from the porous jug generates cold) and the clay itself is a sanitary product used in many water filtering systems, usually in combination with active charcoal. The jugs would seemingly be wasting (evaporation) rather than saving water, but central heating houses and apartments are usually drier than they should be, so a bit of extra moisture is good, and in countries that use chlore to sanitize water, water smells and tastes much better after these gases have had time to evaporate too. For countries that use fluor, although it is also a gas, it is usually used as a stable compound fixed with other chemical elements, so filtering is the only way to remove it, as it will not readily evaporate.