The No. 1 advantage of a king mattress instead of a queen is the additional comfort that it provides. No other mattress has as much room for each sleeper as a regular king-size mattress. A regular king-size mattress has a full 38 inches of sleeping width for each person on the bed. If you share your mattress with someone, this ensures that you both have an additional 8 inches to get comfortable in when compared with a queen bed.
Many king-size beds come with a dual box spring to make them easier to move. This actually makes some king beds easier to move than queens. The mattress, like with a queen, has a certain amount of flexibility, enabling you to move it around corners. A box spring doesn't have any such give, so moving a queen-size box spring is considerably more difficult than moving the two separate halves of a king box spring.
Although there are many schools of thought on children in their parents' bed, the fact of the matter is that it's going to happen sometimes. Children who are sick, scared or who can't sleep often show up in the parents' bedroom. Even if it is just a gathering place for everyone to watch a movie together, a king-size bed provides lots of room for both parents and children.
In addition to a standard king-size bed, there is also another variation on the king mattress available, the California king. These mattresses are 4 inches narrower than the regular king, but come with 4 extra inches in length. This provides very tall people with a mattress that they can fit on comfortably without having to lay diagonally across the bed.