Theseus is a mythological hero, right? Meaning that most people think he's just a legend and never actually existed. The thing is that a lot of people think that myths are symbolic of something that actually happened way back in the day. In Theseus's case, a lot of folks say he represents the civilizing of Athens and Attica, the area around the city. There might not have ever been a guy named Theseus, but there definitely was a gradual shift from a chaotic, loosely structured society, to one that was more unified.
It's pretty likely that at one point in history, Attica was totally wild and lawless. Over time, the area around Athens came under the control of Athenian law, and all the craziness settled down a bit. There might not have really been a bandit like Sciron who fed people to a giant turtle, but there almost definitely were a bunch of thieves running around and robbing whomever they pleased. So, when we hear stories about a hero slaying superpowered villains, it might be a way of representing a much more gradual shift toward order in Athenian society.