Far too often I see girls pounding away on the hip adduction machine in high hopes, I presume, of either "toning" or slimming down their inner thighs. This awkward machine does work and strengthen the adductor muscles which is an important stabilizer muscle for the hips and knees. However, you will not build size and you will definitely not reduce fat in this area. It might sometimes be useful for rehab but most of us have no real use for it.
It may even cause muscular imbalances and lead to injury. Being a stabilizer it needs to be strong, but not too strong. The quads and hamstrings around it are the prime movers and the ones that need to be powerful. So quit wasting your time!
Furthermore, the movement of the hip adduction machine is very unnatural. Instead, try a more functional exercise such as a sumo squat.
The hip abduction machine on which you push outwards, on the other hand, is useful for targeting the glutes. Don't be mistaken, it does not target the outer thighs. What one might consider the outer thighs are actually the outer edges of the quads.