One of the earliest Economic Policy Cards, Caravansaries isn't much good in the beginning of the game, simply because you have few Trade Route Trade Routes and precious few Economic slots in your government (so usually you have better cards to use, such as Urban Planning). However, when you develop the first real governments, and have 3+ Trade Route Trade Routes, Caravansaries becomes very useful for maintaining a full treasury. One of the best things about it is that it affects both international and domestic routes, so it doesn't matter whether you're trading proper with other civilizations, or boosting your own cities - the Gold bonus will apply to all your routes!
Caravansaries becomes obsolete only in the Renaissance Era, so it will be good for a good half of the game.
As caravan routes expanded across the Middle East, Central Asia, and North Africa, the need for places where pack animals and human handlers could pause for rest, shelter, provisions, and safety – for a fee, of course – grew. Public caravansaries were complexes built outside the walls of towns and cities; private ones usually took the form of walled enclosures in the middle of nowhere with storerooms and sleeping quarters, stables, and kitchens. The largest could accommodate hundreds of camels, mules, or horses. The oldest recorded caravansaries were found along the Persian Empire’s Royal Road, stretching some 1600 km (1000 miles) from Sardis to Susa.