Unlike most here, my focus is LOTS of cardio as a triathlete. Fluids with electrolytes are an absolute must for long runs / rides especially. If you sweat out lots of sodium and replace only with water, you can become very hyponatremic (low sodium) in addition to other electrolyte abnormalities (potassium, magnesium, etc). Every year about 10% of people ending up in the medical tent at ironman triathlons is due to free water excess and resultant electrolyte abnormalities. One girl even had a seizure and crashed on her bike at about km 120 at ironman Canada a few years ago because of a sodium less than 120 (normal 140). We'll often use things like e-load and other drinks with a much higher sodium than gatorade. Try it sometime......do a 2 hr run or 4-6 hr ride and your legs will start to cramp terribly, not necessarily from fatigue, but electrolyte imbalance. Take some e-load and you're good to go in 20 min. However, the flip side is that I usually only mix the strongly concentrateed ones to half water / half sports drink, otherwise the high tonicity of the fluids in your gut will retain fluid with them in the lumen of your gut and you'll get diarrhea.
OK, so that was a bit off topic for being in the gym alone, but I think the principles are still there. If you're sweating out sodium and other electrolytes, you must replace them. In the gym for an hour or two you may not notice it and slowly make up the losses with food over the day, but you're still missing them at the time. Same with a sugar source....glycogen only lasts about 45 min, so if you're working out vigorously for longer than that you're going to start to burn fat and even protein (muscle) to make up for the lack of glucose. True though, that if your goal is to burn fat that teaching your body to use fat during long workouts can be aided by limiting your glucose intake. However, at some point your glycogen will still be depleted, even with gluconeogenesis from fat metabolism going on, and you will "bonk" or "hit the wall". At that point there is nothing you can do but take in calories and sit and wait it out.