Kettlebells are a great tool for strength training. Yes, for many exercises they're totally interchangeable for dumbbells or other weights. But for some weighted moves, especially ones that require an explosive movement, kettlebells reign supreme.
Why? The way they're shaped makes them much easier to swing around. You can also hold them by the handle or the bell (the round part of the weight), which allows you to get a different range of motion depending on the kettlebell exercise you're doing.