The head is the most important part of the spermatozoon, as it contains the genetic material of the gamete inside the nucleus. This genetic material or DNA is the sign of identity of the male, which must be combined with that of the female to give rise to the offspring of both members of the couple.
We could say that the head of the sperm serves as a reservoir of the male's DNA and transport of thhis DNA is acheived thanks to the movement of the sperm's tail.