Contrary to popular belief, pre-emergent weed killers don't destroy weeds and their seeds. They simply stop them from growing. Some seeds are known to last 10 - 20 years, so if the herbicide isn't applied each year, the weed will grow.
The question for pre-emergent weed killers is when to apply them.
Pre-emergent herbicides only work if they are applied to your lawn before the weed's growth period. But if applied too early, weather will dilute the herbicide and the weed will grow unencumbered.
Now is the time for pre-emergent application for summer weeds, because usually now is when average soil temperatures reach above 60 degrees. Major summer weeds like crabgrass or clover will only emerge once the soil is consistently over this temperature.