A classic in the Arkham games, Batman's explosive gel is a true staple in his utility belt. You might think it would be useful for simply taking out enemies. But the gel is used to help rid obstacles in Batman's way, as well as help distract, trick, or remove anyone Batman needs gone.
To add to the fun (or perhaps his vanity), Batman always sprays the gel in the shape of his bat symbol. So if you ever cross a goo like substance that looks like a bat? I suggest running really, REALLY, fast.