Weak or untrained stabilizers can be overloaded quickly, sending inhibitory signals to the prime movers of a specific movement and resulting in decreased neural drive to those muscles. In other words, your nervous system will not allow the prime movers to fire at 100% of their capability when they are not protected by the stabilization provided by the machine.
See, your body is smart. It realizes when the structural integrity of the joint over which that muscle crosses is compromised, even if you don’t. So you may be able to perform a squat on the Smith Machine with two hundred pounds. But your brain, just like when you were first learning to drive, won’t allow you to utilize that power when your legs aren’t guided through the motion like the thousands of reps you’ve performed in the past. There are just too many other things going on with which your machine-trained body is not familiar—like gravity, balance, and unguided motion. You simply will not be as strong as you thought. Strength training’s detractors will cite this as evidence that lifting weights is of no benefit to sports performance. And if you continue to lift incorrectly, the only thing you really end up strengthening is their argument.
Go prove it to yourself. After a couple of warm-up sets, do eight reps of a bench press at a weight which makes the last repetition a challenge. Have your training partner spot you to ensure we don’t find your decaying carcass trapped underneath the bar a few days later. When finished, admire yourself in the mirror as you recover and stay loose for your next effort. Now, lie across a physio ball like the subject below (though, personally, I'd advise having the head supported by the ball, too--otherwise you'll overdevelop the SCM's and perpetuate forward head posture) and perform a set of dumbbell chest presses with the same amount of weight.
I doubt you could complete another set of eight. You may not have even been able to get the weight up off your chest. Don’t feel bad. You just received a valuable lesson in neural drive which should feed your desire to train correctly. And if not, you can always get back on one of those machines and feed your ego!