Only a female marijuana plant makes flowers/buds that contain a usable amount of THC. Male marijuana plants only make pollen sacs that they use to fertilize the females. Most growers will throw away any male plants that they encounter to keep them from fertilizing the female plants. If your female plants do get fertilized, they will use all their energy to produce seeds instead of making buds. This is good if you want seeds, but you will run into the same problem since half of the seeds will also be male.
If you would like to start a breeding program to make your own hybrids, I recommend using a method that creates all-female (feminized) seeds so that you don't waste time having to identify and throw out male plants.
Getting clones of female marijuana plants or buying feminized seeds online from a seed bank are other ways you can ensure that all your marijuana plants are female.
If you don't have a choice of seeds, and some of your seeds may be male (like if you just found seeds) than you will want to get your plants to reveal their gender right away so you don't have to waste time and energy on male plants.