The Last Airbender, Aang, was intended to become the Avatar. To put it plainly, an Avatar is a being who has supremacy over the bending abilities of all the elements. As these weights are made of metal, he could have incorporated kettlebells into his Avatar training. This way of training is not just made for people, after all. Any single elemental training routine could have made use of of the weights in one way or another.

While studying water bending, Aang could use water and shift the kettlebells. He could slip them into water and use a selection of techniques to locate the kettlebell at the bottom of the water. He could also fine tune his methods. This could be done by having someone hurl kettlebells at him and him having to understand to use water to defend himself.

As one aspect of his bending training, Aang could easily push away and bend the kettlebells along with the Earth. He could take part in various sessions where he would stop the weight from making contact with the ground. In order to do this, Aang would fire at the ground, shooting away chunks of the surface to deflect said weight. Once he had a firm grasp on Earth bending, he could possibly alter the metal to where another level of Avatar training could utilize such weights.

I believe that the hardest thing for Aang to do in his training would be the sessions involving fire bending. Prince Zuko would have to be the one to dispense such knowledge, too. I believe that Zuko would be apt to firing kettlebells with flames about them at Aang. In order to prove himself, Aang would have to dispel the shots in some way; ideally, he would return fire by reversing the shots back at Zuko. The versatility of these unique dumbbells cannot be dismissed and it's one of the many reasons why fitness authorities such as Lorna market them to the public.

Kettlebells are a versatile tool to work out with, and that's just from the ordinary human perspective. If it were from the standpoint of someone who is in training to emerge as the next master of the elements, it is truly useful. Keep in mind that the Avatar could use a simple kettlebell in his training to someday attain mastery over the elements. You can think about what a mere human could do as well.