The relationship between structure and function is an underlying theme in biology. Determination of the structure of biomolecules has allowed scientists to develop processes that exploit their structures.
Thanks to Watson and Crick (and Rosalind Franklin, but that's for another day), we understand the structure of DNA and base pairing. Many of the techniques described here rely on the rules of base pairing. DNA sequencing, Northern and Southern blots, PCR and RT-PCR, DNA microarrays and in situ hybridization all use the principle of base pairing to identify the molecule(s) of interest.
Know that good feeling we get when we work out? That feeling starts when natural endorphins bind to their receptors. The endorphin receptors recognize the structure of endorphins, bind to them and start a series of events leading to a change in gene expression.
Opiates like morphine and heroin can cause that same good feeling. Guess what? Their structures are similar to endorphins. They are recognized by endorphin receptor and cause the same feeling.
Knowing the structure of a protein and the individual amino acids that make up the protein, you can use in vitro mutagenesis to create new proteins with different properties.