The AT-format is a standardized form factor, which determines how large the main board, how wide must be the spacing between holes and where all components are placed. In addition to the AT format, there is a smaller version, the Baby AT format, which was developed somewhat later.
AT power supplies are directly connected to the 230-volt main line. So that it can only be turned off totally and there is no standby mode. Modern formats, in this case ATX, has main connections to the motherboard, allowing a software controlled standby mode. The advantage with the AT power supply, however, when the condition is turned off, no power is consumed.
With the old computers, the device was turned off directly at the AT Power supply. Only by adding housing, the switch is mounted on the front. This make a 230-volt cable had to be led through the housing. Due to the various switches and cables that were not standardized, it was difficult to replace a power supply.