it's basically just simple sugars that'll give you a short burst of energy when energy levels are low on longer duration runs etc... you'd never notice a difference in short distances but in marathons etc it would probably mean the difference between hitting 'the wall' and not... which would have a massive effect on your finish time.
In reference to the last comment, I should add that I don't mean energy tablets must be eaten during a marathon, you could eat a whole number of substitutes (gels, jelly beans etc), but some form or nutrition/energy source in general would be desireable to avoid hitting the wall.