The sperm cell must be propelled to the egg cell so that fertilization can occur. For organisms that live in water, the sperm will use their flagella to swim through the aquatic environment to the egg. For land organisms, sperm cells will fertilize the egg inside the female reproductive system and still must use their flagella to propel themselves toward the female gamete. Locomotion requires energy and this energy can be supplied by mitochondria which can carry out the process of respiration. The resulting ATP can be used to perform cellular work such as locomotion.
The head of the sperm cell contains the haploid chromosome complement for the species in question. There is a region that contains enzymes to help the sperm cell penetrate the surface of the egg. The midpiece contains multiple mitochondria which will be able to carry out respiration to supply the tail with the energy supply it needs to deliver the sperm cell to the egg.
During fertilization, only the sperm's head containing its DNA will penetrate the egg and the two nuclei fuse. However, the midpiece and tail will detach. Because of this, any mitochondria in the resulting offspring originated from the mother's egg and thus the circular mitochondrial DNA and all of its genes pass from mother to child.
To conclude, mitochondria are organelles necessary for the process of cellular respiration. The sperm's midpiece contains many mitochondria so that a supply of energy is available for the sperm to perform its function of traveling to and later fertilizing the egg.