I just looked it up in the manual:
Laser Range Finder and Designator (LRFD) The TADS Laser Range Finder Designator (LRFD) can provide both range to and designation of a target. In both instances, the aircraft must be armed to operate the LRFD. To range a target, the TEDAC RHG LRFD trigger is pulled to the first detent and three ranging pulses are emitted. To designate a target, the TEDAC RHG LRFD trigger is pulled to the second detent and a continuous laser pulse is emitted. Pulling the LRFD trigger to the second detent also engages the Target State Estimator (TSE). General guidance with regards to the laser are: If neither the target nor the aircraft is moving use first detent ranging, otherwise use continuous second detent designation to employ the TSE.
In short it helps you to target targets who are on the move and/or whille your aircraft moves.