I'm assuming that it's the glucose part that you're asking about, and not the electrolyte replacement issue. It's been quite a while since I studied biochem and physiology, but here goes...

First, you might think that the water you drink is somehow absorbed directly by your body, like sucking it up through millions of tiny straws, but that's actually false. Actually, your body does it indirectly. Water is passively drawn out of the interior of your small intestine and into the tissue by making the tissue of the small intestines "saltier". Then it is absorbed into the capillaries and into the bloodstream. Lining your small intestines are millions of tiny pumps that pump sodium molecules into the tissues thus making them "saltier". The thing is, the pump requires a molecule of glucose to tag along at the same time. No glucose, no pumping action. Similarly, no sodium, no pumping action, too.

Glucose is normally not found in food or beverages. Instead, bigger, more complicated sugar molecules are broken down by enzymes in the small intestine to produce glucose. But that takes a bit of time, so you can see the hypothetical advantage of having glucose in a rehydration solution.

You can make your own poor man's sports drink by mixing 5 tablespoons of table sugar and a third of a teaspoon of table salt (be careful if you're on a sodium-restricted diet) in a liter of water. Don't store longer than a day, even if chilled. That sugar water solution is a prime breeding ground for bacteria.

I'm not a doctor, so if you have any questions about your specific situation, please consult your healthcare provider.