Few tourists making their first trip to France go home without having seen Versailles. But why do so many want to see Versailles in the first place? Yes, its history goes all the way back to the 1620s, with its comparatively modest beginnings as a hunting lodge built for King Louis XIII, but much in Europe goes back quite a bit further. It did house the French royal family for generations, but absolute monarchy hasn't been a favored institution in France for quite some time.
Only the most jaded visitors could come away unimpressed by the palace's sheer grandness, but those in need of a hit of ostentation can always get it on certain shopping streets in Paris. The appeal of Versailles, and of Versailles alone, must have more do with the way it physically embodies centuries of French history.
You can watch that history unfold through the construction of Versailles, both exterior and interior, in these two videos from the official Versailles Youtube channel. The first begins with Louis XIII's hunting lodge, which, when the "Sun King" Louis XIV inherited its site, had been replaced by a small stone-and-brick chateau. There Louis XIV launched an ambitious building campaign, and the half-century-long project ultimately produced the largest chateau in all Europe.
The Sun King moved his government and court there, and of course continued making additions and refinements all the while, extending the complex outward with more and more new buildings. Louis XIV's successor Louis XV put his own architectural stamp on the palace as well, subdividing its spaces into smaller apartments and adding an opera house.
But when the French Revolution came in 1789, the royal family had to vacate Versailles tout de suite. Then came the removal of the absolutism-symbolising "royal railings" out front, the taking of its paintings that hung on its walls to the Louvre (the third most popular tourist attraction in France, incidentally, two spots ahead of Versailles), and the auctioning off of its furniture. While the anti-monarchical fervour of the period immediately following the revolution wasn't particularly good to Versailles, later rulers implemented restorations, and the current Fifth Republic may well have spent more on the place than even Louis XIV did. And so we have one more reason six million people want to visit Versailles each and every year: they want to see whether France is getting its money worth.