They have their purpose (dynamic cardio) but if you want strength, you lift. If you want functional strength, lift explosively.
Go ask the gold medal winners in Olympic Lifting about their training or the record holders in powerlifting about their training. Or for that matter go take a look at your college and professional football teams. I don't think you'll see kettlebells in their day to day training. You may see them from time to time, again as a form of cardio or conditioning...but not as a strength tool.
KB's can give you a different stimilus, which can produce results....but if you've never really lifted before then your time would be best spend under the iron. If you're not improving under the iron then your program needs changing. Most people stay with the same general exercises in the same rep range. Go do some singles or triples and I bet you will get stronger.
And no, not every piece of eqiupment has a use...unless that use is to sell you something.
Again, use them for cardio, not strength.
The term "functional strength" has been bastardized to hell in the fitness industry. So much so that people think doing lifts on a damn bosu ball or swinging crap around is functional.
You know what the two most functional exercises are? The squat and deadlift. Anytime you pick something up off the ground you are doing a deadlift. Anytime you sit down and up from a chair you are doing a squat. With all of my elderly clients we do squats. A couple months later they are getting up and down with fewer problems. That's an increase in functional strength.
Functional strength for an athlete is strength in the specific range of motion for that sport. With running you have opposing left/arm movements. So, step ups with an opposite hip extension prove beneficial as do lunges with a cable row. BUT, you need general strength before you need functional or specific strength. So general strength lays the foundation for functional or specific strength.
Here's an idea for increasing strength that has stagnated:
Clusters: Perform one rep at a weight you could lift 2-3 times. You can also lessen the intensity by performing 2 reps at a weight you could lift 4-5 times. Perform 5 clusters with minimal rest (5 seconds, 10 seconds, whatever you need, experiment)
So a set of clusters would look like 1-1-1-1-1 or 2-2-2-2-2. If you can easily get to the 5th rep then either your rest is too long or your weight too light.
Do 3 sets of clusters for each exercise, 4-5 exercises. 3 minutes between sets.
Alan