It's a way of measuring the "health" of some environments. If you take a stream impacted by dumping, and compare it to one that's not, but has the same water chemistry and physical features, you might expect that there would be fewer (and fewer types) of organisms living in it. But not all types of pollutants are obvious just by looking at the stream, or they might only happen periodically. Looking at what lives in the stream, especially the organisms which are less mobile (say snails instead of fish), it will tell you if the conditions were good over a period of time.
And biodiversity can be used as a measure of sustainability. Think of a food pyramid - there are more prey organisms than there are predators, and few predators just eat one food item. The more "kinds" of prey that there are, the more likely the predators above it will survive if any one of the prey dies out (through elimination of its food source, trapping, poisoning, etc.).
Biodiversity has other benefits too - from aesthetics (imagine a forest or field with just one kind of plant!!), to more potential plants for food and medicinal properties.
And biodiversity can be used as a measure of sustainability. Think of a food pyramid - there are more prey organisms than there are predators, and few predators just eat one food item. The more "kinds" of prey that there are, the more likely the predators above it will survive if any one of the prey dies out (through elimination of its food source, trapping, poisoning, etc.).
Biodiversity has other benefits too - from aesthetics (imagine a forest or field with just one kind of plant!!), to more potential plants for food and medicinal properties.