Typical applications occur in the early part of the year before warm-season weeds appear and in autumn for cool season weeds. These preventative herbicides actually can be helpful throughout the majority of the growing season. Various weeds germinate at different times, so repeated applications of the correct chemical can eliminate the majority of the weeds in any lawn or yard.
The weed is eliminated permanently, because the seed is not allowed to germinate, and the young tender sprout is killed. A PreEmergent is activated in the top layer of soil, where the weed seeds sprout, and by design, it stops the plant from growing. This is one key advantage over just putting down new soil or mulch to bury the seeds. Weed seeds remain viable many years, waiting for a chance to pop up. Pre-emergents break the cycle of constant regrowth of new weeds.
The only Down Side to a pre-emergent is you Do NOT plant new grass seed. Most products are non-selective, meaning they could be called Seed Preventers instead of Weed Preventers. Most typical lawn grass seed will not grow, so you must calculate the timing of an application to allow a window for starting a new lawn or reseeding a meager lawn. The label should advise you.