ADDUCTORS is a fancy word for inner thighs, and they are small muscles compared to the other muscles of the thighs, but they are indeed important.The adductors are responsible for the motion of bringing your leg back in towards your body, and also for crossing your legs. Think of a jumping jack. Your legs are apart, then together. The motion of bringing them together uses your adductors. Now, do another jumping jack, but do a crossover with your legs. This also uses your adductors.
You won't get a lot of "definition" or "toning" from strengthening your adductors, but you will gain strength which will translate directly into a stronger hip/pelvis joint and knee joint, which can prevent injury or pain in either the hip, knee or groin.
It's important to keep this muscle group stretched as well. If you think of all the adductor muscles as rubber bands, connecting hip/pelvis to the knee, and if the rubber bands get tight, the potential is to pull the kneecap off track, which can create pain.
So, stretch, strengthen and train you adductors!