Julius Caesar is historically significant mostly because he was the main driving force behind the demise of the Roman Republic and the creation of the Roman imperial system.
Caesar was born in 100 BCE and was killed in 44 BCE. This was at a time of great upheaval in the Roman Republic. For instance, it was in 82 BCE that Sulla took control of Rome and instituted his reign of terror. From then on, Roman politics was in a state of constant conflict and even civil war.
Julius Caesar tried to end this chaos. Eventually, his solution was to take power for himself. It is hard to know from this far in the future whether he was really power hungry or whether he was actually motivated by a desire to make Rome more stable.
Whatever his motives, Caesar's actions led to the end of the Roman Republic. This is what makes him most historically significant.