When you run, your body uses energy. The question is whether you need to replace this lost energy during your run. And that really comes down to a) how much energy you have in your body and b) how long you're running for.
How long are you running for?
If you're going for a run for longer than 90 minutes then you should be thinking about taking on energy during your run so that you can maintain a high level of performance. You can do this through an energy drink, gel or through food. The issue many people face by eating during a run is that blood is directed to your stomach for digestion and you can counteract any of the benefits of fuel intake by your body needing to digest the food. This is why people choose energy gels, as they have the ingredients that the body needs whilst being easy to digest.
Energy Boost
The other thing that you might think about is your energy levels. If you're feeling tired or sluggish you can kick-start this by taking on caffeine. This is a stimulant, which means that it can temporarily increase the amount of energy available to you. Energy gels with caffeine are designed just for this so can be an effective way of giving energy to your muscles. You do need to be careful with caffeine though, and your body might react differently to this, so it's a good idea to try these before you commit to using one on race day.
How many gels should you take?
The general rule is to take one before you start your run and then one every 30 minutes after this. So if you're doing a park run/5k you shouldn't need any whilst you're out, 10k you might want to take one after 30 minutes (it really depends on how far to the end you are whether it's worthwhile) and a half/marathon you probably want to be working this out as part of your nutrition planning.
In summary
Energy gels are designed to increase and maintain performance during your run and are used by athletes at all levels. Their popularity is growing and increases to grow as people are more aware of their hydration and fuel plans as part of their running strategies.