The most obvious way to reach the area where a glass panel needs to be replaced is by using a crane from ground level and/or a mobile elevated working platform (MEWP).
A crane is able to lift heavy glass panels to the desired level so workers can install the window, possibly hanging suspended alongside the facade with a suspended scaffold or by using a MEWP.
This equipment will do the trick, but cranes on ground level do have their disadvantages: first, they need to be parked somewhere. High-rise buildings are often situated in crowded areas and to park a crane somewhere might only be possible when the street is (partly) blocked. Also, the costs of hiring heavy machinery are high, so when glass needs to be replaced from time to time, large expenses have to be made.
Another important factor is that cranes and MEWPs can only reach to a certain height, making it impossible to replace windows, more than a hundred meters high.