Ok let's do this again...
99% of you guys are working out in the afternoon or evening.
Pre-workout products makes you feel more "powerfull" and awake. Of course since it's full of caffeine and taurine, but sold to you at a higher price than intakes you could take from coffee or tea or even some so called energy drinks (energy drinks of course are total crap and you should avoid these any day of the week).
So let's say your hearth is at 40% while beginning training all natural. With a pre-workout you'll jump at 70% without doing anything. (just example numbers). You have less range with your hearth with a pre workout. You are already pumping up, so unless you have a superman hearth, you should hit your maximum with less effort since an external substance is already raising your heartbeat.
Now, you finished training, around or after diner (souper) but you still have at least 10 hours of remaining effects of your pre workout in your system. Where does that lead you? In your sleep of course... while most of you junkies are pumping caffeine daily enough to be resistant and be able to sleep on the effects, caffeine still prevents you from getting a deeper sleep. Deep sleep is the only way to generate muscle while sleeping, so you are just fooling yourself into productivity.
Also, pre-workouts, with stimulants, tend to lessen your appetite. Again a counter productive effect.
And there's the few other side effects of course..
Just eat right, stop the sugar, salts and cut the fat stuff and get your mental do it's job while training.