Leading edge flaps add lift in slow-speed high-alpha situations.. takeoff and landing.
As these two areas are fraught with peril for a model.....
as well as being difficult to fabricate..
Unless you can have a flight control computer as in the full-scales control the deployment of these things (it monitors angle of attack, airspeed, g, and airplane weight), it's better to takeoff and land fast, with the leading edge flaps artfully represented by panel lines.