Edited sentence: In some cases, exceptionally tall skyscrapers have been built not out of necessity, but to help define the city's identity and presence or power as a city.Not only do they define the skyline, they help to define the city's identity.They are built not just for economy of space; like temples and palaces of the past, skyscrapers are considered symbols of a city's economic power.Today, skyscrapers are an increasingly common sight where land is expensive, as in the centres of big cities, because they provide such a high ratio of rentable floor space per unit area of land