Men produce millions of sperm daily, unlike women who are born with all the eggs they'll ever have. The cycle of sperm regeneration takes approximately 74 days on average, so this should be taken into consideration when trying to conceive.
Sperm are manufactured in the testes in the seminiferous tubules, and after their heads and tails are developed, they move to the epididymis to mature further. They are stored there until they are ready to be expelled from the body. During ejaculation, the sperm are pushed up the vas deferens, a tube which loops around the bladder. The ends of the vas deferentia (plural of vas deferens) are known as ejaculatory ducts, and here, the sperm mixes with semen from the seminal vesicles. The ducts lead to the urethra; the prostate adds its secretions to the fluid at this time. The Cowper's glands, or bulbourethral glands, also add secretions. The semen with all the sperm and secretions is then ejaculated through the rest of the urethra and out into the woman's body.
Healthy sperm are needed for conception to occur. Sperm are regenerated in a constant cycle, giving each ejaculation fresh, healthy sperm so as to fertilize an egg.
Since sperm are also cells of the body, they receive the same nutrients and toxins that are put into the body. If a man is taking care of himself, eating properly, not smoking or using drugs and getting the right vitamins, his sperm will most likely be healthy, unless there is some kind of underlying problem. However, if a man does partake in unhealthy activities, the sperm will also be unhealthy. The fix is simple when infertility is related to an unhealthy male lifestyle: Stop the problematic habits and the sperm will become healthier. It will take 74 days or so for this to occur, since new sperm take about this long to go through the cycle and regenerate.
The benefits of waiting for sperm to regenerate before trying to conceive are numerous, but the main reason is to allow a healthy sperm to fertilize the egg, giving it the best chance of survival. Defective sperm can lead to a defective embryo, where there are chromosomal abnormalities or birth defects. Many zygotes with chromosomal abnormalities are spontaneously aborted (miscarried) since they have no chance for survival outside the womb anyway. This is why many couples choose to use birth control for three months after a man has made significant lifestyle changes, like quitting smoking or drinking alcohol.
It is often said that sperm take three months to regenerate, but 74 days is only two and a half months. Still, this may be to account for individual differences.