Why is Lucozade better than water for marathon?

Here is the ingredient list of Lucozade from their website:

Carbonated Water, Glucose Syrup (26%), Citric Acid, Lactic Acid, Flavourings (including Caffeine), Preservatives (Sodium Benzonate, Sodium Bisulphate), Antioxidant (Ascorbic Acid), Colour (Sunset Yellow).

Sugar content of Lucozade: 68 grams (per 12.85 ounces).

As you can see, there is an awful lot of sugar in that drink - there are 4 grams per teaspoonful, so this drink has ...

... wait for it ...

17 teaspoonfuls of sugar in each bottle!

There is also 46 milligrams of caffeine in each bottle - this is about the same amount of caffeine is a good cuppa, but less than a Starbucks Espresso.