The Fifth Dimension was the home of Mr. Mxyzptlk and other beings of incomprehensible power like him.
History
The Land of Zrff tribunal.
The council of the Land of Zrff had strict rules about interfering with other dimensions. Mxyzptlk was slapped down for his dirty deeds in Superman's dimension and was required to do three months of community service in the third dimension, without his powers, to make up for his misdeeds.
It was only possible to move between the Fifth Dimension and other dimensions during periods of dimensional interface, which occur roughly every 90 days.