You don't normally because they aren't for hydration at all other than there is water involved. The history of their development coming out of the Florida Gator football program is quite interesting.

When you sweat you lose more than water. If you've tasted sweat you probably noticed it's salty. So after exercising for long periods you need to replenish more than water to keep your body balanced. That's where electrolyte drinks can help, providing a lot of those salts and minerals as well as some water.

There are a ton of caveats though because they don't hydrate you. In fact, it is my understanding they can be dangerous if you're actually dehydrated. People experiencing heat exhaustion or dehydration should only drink water, recovering their electrolytes only after they recover.

Water is the primary thing you should be drinking to hydrate yourself. If you're doing light exercise and feel like it would help, it's probably better to water it down and focus on more water.