There are many reasons why you might want to train your large dog to use an indoor litter box. Some dog owners are gone for long periods of time and don't want to run the risk of even their large guy have an indoor accident. Your adult dog large might feel as if he'll be punished if he has an accident indoors. With a dog litter box, you can give him the opportunity to relieve himself while you're away from the house. He will be able to go potty while keeping a clean house and staying out of trouble. If you know you're going to be gone for long periods of time, longer than your dog can handle holding it, setting up a large litter box for your dog is not hard to do.
Don't think of a large dog litter box as the same as a cat litter box or a small box that you may use for a small breed dog. He just needs a space where he knows he can go without getting in trouble to relieve himself when he's unable to go outside. You can also use newspaper to line a box that your large dog can step into or set up a litter box using a tray liner or crate liner and a thin layer of cat litter. Potty training your large dog to use a dog litter box is not much different than house-training him. It's going to involve a lot of repetition, reminders, and rewards.
If you choose to train your large dog to use a litter box, be sure you have it set up and ready to go before you begin training. It will be best if you pick a confined area within your home that your dog can rely on anytime he needs to go. This means the area needs to be kept clean and cannot change from day-to-day or week-to-week. Be ready for some potty training exercises and lots of rewards for making it to the litter box and not having accidents in the house during training and while you are away from the house.