Preemergent, also known as pre-m, is designed to prevent seeds from germinating and growing. Pre-m is generally applied in the spring to prevent weeds from taking hold early in the season. If you are able to prevent a good number of weeds from starting in the first place it will make weed control later in the season easier to deal with.
The residual action of most preemergent products lasts anywhere from 60-90 days. Some companies will apply two preemergent applications to extend its control into late spring and early summer. If you are considering aeration it is a good idea to do it before your final preemergent application. The soil plugs that are pulled during aeration will punch a whole in the surface barrier that is designed to prevent weeds from emerging.
Normally preemergent is no longer applied once ground soil temperatures surpass 60 degrees. This may be anywhere from around May 1st to June 1st approximately in northern Utah.
Preemergent is especially helpful in preventing crab grass. Crab grass is an annual plant that can be very difficult to control once is has taken hold in the turf. Crab grass requires specific chemicals for its control once it has emerged.