Feminized cannabis seeds grow flowers more easily and with less downtime. Since every seed is a female, there is less garden waste when you grow cannabis from feminized cannabis seeds.
If you grew buds before this great innovation in horticulture, then you had to germinate, plant and monitor your plants and then destroy them if they produce pollen sacks and are male. Otherwise, your male seeds would pollinate your females, resulting in a fraction of the potency compared to a flower germinated and grown from feminized cannabis seed.