The Church was built by emperor Constantine on the property that used to belong to a wealthy Roman family called Lateran, which donated it to the Church. It was consecrated by Pope Sylvester I. The basilica was additionally dedicated to Saint John the Evangelist and Saint John Baptist in the 10th century and thus it is frequently called Saint John in Lateran.
Dedicating one church to various saints is not permissible nowadays, unless they typically create a pair (e.g. Cyril and Methodius).
The Church used to be the actual seat of the popes until around the 16th century, when they moved to the Vatican and it still continues to be the Cathedral of the Pope. Its universal celebration is a sign of love for and union with the Seat of St. Peter. Pope Clemens XII (1730-1740) called it Mother and Head of all the Churches in the City and in the World.