The base pairing rule ( a is to t and c is to g), allows the DNA to replicate itself with accuracy. It's important because some, such as single cell molecules, depend on the survival of their species on fast and accurate replication.
It can be unzipped by RNA polymerase and make corresponding RNA that maybe translated proteins required by the cell. This is important to keep the integrity of the DNA as the basis of heredity but at the same time produce the products that are encoded in the genes.
