Most people are not familiar with transfer cases, but if you have a 4-wheel-drive vehicle, you have one. Depending on your vehicle, you might use a shift lever to access 4-wheel-drive, you may push a button on the dash, or transitioning to 4-wheel-drive might be automated. The transfer case makes sure you have sufficient allocated power available between the front and rear axles. When not in 4-wheel-drive, if the vehicle is rear-wheel-drive, power goes to the rear wheels until you need 4-wheel-drive. That is when the transfer case steps in and transfers some of the power to the front wheels.
The transfer case is serviced by periodically draining the fluid and replacing with fresh fluid. We can also check for leaks and damage. Transfer case fluid cools and lubricates the gears, chains, bearings, shafts, and other internal transfer case parts. Over time, the additives in the fluid decay and discontinue protecting. Bits of metal and clutch material can wear off and contaminate the fluid. There is no filter in the transfer case, so if the contamination stays too long, further wear can accelerate.
Your owner's manual may not have a recommended interval for when you should change your transfer case fluid, so ask your service advisor. There are several things that affect how often you should change the fluid, so tell them how often you use 4-wheel-drive and if you drive in wet environments like crossing streams or through mud and snow.
Transfer case parts will eventually wear out and need repairs, but properly servicing your transfer case can keep that day as far in the future as possible.