Why do animals have different structures that perform similar functions (e. g. gills and
lungs)?

Animals have different structures (like gills, lungs) to perform similar functions as a mechanism of an evolutionary continuum with adaptations depending on various limitations like phylogeny, structure and behavior of animals, and other environmental factors.

Explanation:

Respiration in any organism takes place through the exchange of gases. Respiration differs in different animals and it takes place through;

Gills are specialized structures in fishes which absorb the oxygen dissolved in water and lets out carbon dioxide into the water. Later gaseous exchange takes place through blood vessels in the gills.

Animals use lungs for respiration by inhaling oxygen and exhaling carbon dioxide. The gaseous exchange occurs in the alveolar sacs of the lungs.