During the long hot dry summer months the scarcest resource for many of the larger animals is simply water to drink. The positioning of ponds throughout the garden ensures that this need is taken care of.
Water is also an important habitat for many insects to live out a particular phase of their lifecycle. For example the dragonflies can spend up to three years living under the water as a nondescript nymph, before they emerge as the magnificent insects we know.
And finally for us as humans the presence of water has a calming and soothing effect. A bench by the pond is a wonderful place to sit and let the mind quieten.