Power Control in CDMA
The various codes in each handset are not fully orthogonal so some interference occurs between the received signals at the base-station.  The signal from the handset varies with distance from the base station and there are other real-world imperfections such as multi-path, building attenuation, intermodulation etc.

The range of those attenuations is more than the signal to interference ratio of the coding so it is useful to adjust the power of the handset to normalize the signals at the base station so that there is a lower ratio between the weakest handset signal and the strongest.