DNA is made of two very long chains of smaller molecules called nucleotides. The chains wind round each other to form a double helix. Most chains contain only four kinds of nucleotides, A, T, G and C. The sequence of the nucleotides in the chains forms a code that specifies the entire animal, defining its particular and its general characteristics. A triumph of modern biology has been to work out how the DNA sequence (the "genetic code") is translated to produce a particular animal with its own shape, anatomy, metabolism, habitat, behaviour and so on. The genetic code in human beings is contained within chains of three thousand million nucleotides. In other animals the chains are even longer, an astonishing amount of information. It would be a crime not to save this vast store of knowledge.
An important property of DNA is that millions of accurate copies can be made within just a few hours, using the process known as PCR. PCR machines, and the chemicals needed, are now relatively cheap to buy. With the amplified DNA produced by them, we can make other kinds of molecules that occur in the animal. In principle, under the right conditions, we could make any of its molecules. Animals often contain substances that are analogous to those causing human diseases, or that are valuable in themselves as drugs for treatment. It could become necessary to make them after the animal has gone extinct. In the future it might even be possible to reconstruct entire animals from their DNA sequences.
Although a lump of frozen tissue is very useful indeed for saving DNA, a sample of frozen viable cells can be even better. A living cell contains not only the DNA but also the structures and chemicals that help it to function. Cells can be cultured and then frozen in special conditions so that they return to life when they thaw out, but this process only works for some cells and some species, and it is a little more complex and expensive. Where we can, we will freeze viable sperms and eggs to be used for fertilization, producing new young animals. When species are endangered but not extinct, artificial fertilization can counteract the loss of fitness through inbreeding that happens when numbers become low. Thus the possession of frozen gametes can dramatically help efforts at conservation.