The biggest and probably most obvious advantage of a king-size bed is the space. The space is especially helpful if you have a partner in bed with you; you can both spread out as much as you want. Usually, king mattresses are good for people with a bad back, since they can sleep in any position they want.  King-sized beds are also good for those with larger bedrooms. Queen beds look too small in larger rooms. 
There are also some disadvantages of king-sized beds. For starters, they are usually more more expensive. Not only are the mattress themselves more expensive, but the frame, box spring and sheets for these beds are pricier, as well.  If your bedroom is on the smaller side, it may be hard for a king-size bed to fit. Even getting it in your bedroom can be difficult.