They really are fun tools that open up workout opportunities since, as George points out, you can use them for strength, but with a light enough version, you can do major movements with big muscle groups in a way that'll sustain some cardio, too. Certainly, it's not a cardio workout like swimming or running, but sets of squats or overhead presses with a kettlebell will get the heart rate up in a great way. There are lots of YouTube videos of exercise routines that can really help with functional strength and fitness.