Even though you may not use your transfer case all the time for 4 wheel drive the transfer case is still spinning and needs proper lubrication to make the transfer case last.
So if you do not change the transfer case fluid out often enough then the transfer case fluid may become contaminated and cause problems in your transfer case.
That could then eventually lead to the failure of your transfer case and possibly need a rebuild of the transfer case or replacement which can be costly.
If you have a transfer case on your vehicle you should change the transfer case fluid every 30,000 to 60,000 miles.
Along with that you should also change your transmission fluid and transmission filter after every 30,000 to 60,000 miles as well.
Changing your transmission fluid and filter as well as your transfer case fluid will extend the life of your transfer case and your transmission which saves you on costly repairs of your vehicle.