Why are testicles necessary to have?

Not necessary: Testicles function to produce two important things: sperm and testosterone. But neither is necessary for life. Case in point: many men have a vasectomy to prevent the escape of sperm (contraception). And many men with advanced prostate cancer are treated with blockade of testosterone or even removal of the testicles.

Sperm & male hormone: Testes produce both. You need testosterone to be a complete man, it fuels puberty, made your penis and testes enlarge & your beard & pubic hair to grow. Sperm as essential if you want to have kids. That is why men need testes. Without them a man is a eunoch.