Ancient empires or cultures, lost and forgotten but even simply decadent ones, are an important part of any fantasy world.
On Oerth we can find Suel Imperium, Baklunish Empire and the decadent Great Kingdom, on Toril Netheril and Myth Drannor among the others, on Aebrynis the Adurian Empire, the decadent Anuirean Empire and the lost Masetian culture, on Krynn the lost Kingdom of Istar, on Mystara the lost Nithian and Milenian empires and so on..
They are all sources for many adventures and plots in your campaigns.
Ancient ruined cities, forgotten dungeons, abandoned castles or towers are perfect places for your adventures and your dungeons.
You can also introduce different features in your world: for example you can place ziqqurats and pyramids in a medieval setting.
Books, coins and art objects of an ancient culture are good examples of treasure your party can find.
Such things add realism to your campaigns.
Ancient lost spells, powerful magic items, powers forgotten and buried for a very long.
I'm sure you know what I'm talking about: this is a perfect way to introduce new spells or magic items.
THE LOST WORD
The Empire had disappeared after a war, a natural (or magical) disaster, or a plague.
Its ruins had been swallowed by jungle, forest, desert sands or by the sea.
Some descendants of the empire's population still dwell near those ruins or in coastal areas.
They are savage cultures that consider the ruins holy but haunted places.
THE EMPIRE STRIKES BACK
The Empire is still alive and now on the throne sits a capable leader who wants to bring back the empire to the ancient glory
He's going to start a war to win back the lost regions, using all the ancient empire's powers: magic, armies, powerful weapons, special monster such as dinosaurs, undead creatures, demons and many others.
THE GAME OF THRONES
The empire is still alive but is torn by civil wars, internal struggles of different powers that fight for the throne.
Examples are the Great Kingdom of Aerdi after the Greyhawk Wars or the Anuirean Empire after Roele's death.
Such a scenario is well suited for political struggles, large scale battles, nobles murdered or poisoned, secret plots and many other things.
Introducing in your campaigns an ancient empire increases the level of details of your world and guarantees you many hours of adventures.