And it came to pass in those days, that there went out a decree from Cæsar Augustus, that all the world should be taxed. (And this taxing was first made when Cyrenius was governor of Syria.)  And all went to be taxed, every one into his own city.  And Joseph also went up from Galilee, out of the city of Nazareth, into Judæa, unto the city of David, which is called Bethlehem; (because he was of the house and lineage of David:)  to be taxed with Mary his espoused wife, being great with child. - Luke 2:1-5

Caesar Augusts (Gaius Octavius–Julius' great-nephew in history books) appointed a man named Cyrenius (Quirinius) to gather a census and collect taxes in the area of Syria. This area, under Roman rule, included Judea as well.

Luke includes this information for us to explain why Mary did not give birth in her home town of Nazareth.

So, why is that so important? Couldn't Jesus been born in obscurity in Nazareth just as easily as Bethlehem?

Of course, He could, but there is a prophecy of His birth that must be fulfilled.

But thou, Beth-lehem Ephratah, though thou be little among the thousands of Judah, yet out of thee shall he come forth unto me that is to be ruler in Israel; whose goings forth have been from of old, from everlasting. - Micah 5:2

Just look at the amazing providence of God in this little bit of information. The prophecy said that Jesus was going to "come forth" out of Bethlehem. But Mary lived in Nazareth, and poor farm girls didn't do much traveling. Neither Caesar nor Cyrenius had any interest in the Savior. They were simply collecting taxes. But God, in His providence, allowed this tax to take place in just the right way (by city of lineage) and at just the right (just before Mary was to deliver) time so that Mary would give birth while in Bethlehem.

Amazing wisdom and providence!