The prostate is not really a bad gland; It's a good gland in a bad place. Think of a donut around a water hose and you'll have a good model for the prostate (doughut) and the male urethra (the hose), through which urine must flow to exit the body.
The prostate contributions nutrients and most of the liquid to the male ejaculation, so it plays an important role in reproduction. Unfortunately, the prostate tends to continue growing through adulthood, especially after the age of about forty. This enlargement is not life threatening, but it can diminish your lifestyle.
Imagine closing down the nozzle on your garden hose so that there is a trickle instead of a gush. It will take a lot longer to wash the car with such a small stream. Same thing is true when emptying your bladder if there is substantial constriction at the donut hole.
Why does the prostate get unruly?
As a man ages, he produces less testosterone in general, but because his levels of several other hormones increase, he actually winds up with an increased level of testosterone in the prostate gland. There it is converted, via an enzyme called 5-alpha-reductase, to dihydrotestosterone (DHT), an even more potent form of testosterone.
Because of elevated levels of estrogen with advancing age (yes, men have estrogen, though in far smaller amounts than women), DHT elimination is slowed. The accumulating DHT is what stimulates prostate growth.
Some men try to treat the problem the way men often treat medical problems: by ignoring it.
Not all big prostates cause problems; Not all small prostates are trouble free. Some of it is just luck of the draw. Think of that dough again. It can get bigger in a couple of ways. This inside of the donut could grow inward, pinching the hose. Or the outside could grow outward, affecting nothing. So it is with the prostate. If you're unlucky enough to be an "inny," you'll be in a pinch.