Lateral exercises involve lifting your extended arm upward in a wide arc. In order to work all three heads, you need to do Laterals to the front, to the side, and to the rear. When you do Laterals, you do not involve any other muscle groups. You have to completely isolate the various heads of the deltoids. Because you are isolating the deltoids, you cannot lift as much weight as with pressing movements.
Helps bring out definition in the shoulders and works the side and rear deltoid heads together.
Also, helps to develop the traps. Helps to create separation between the deltoid and the pectoral muscles. Doing them seated allows you to do a stricter movement than when standing. Bending forward from the waist about 45 degrees. To work the rear head of the deltoids.