Why is this the power symbol?
The well known on/off power symbol was the result of the logical evolution in user interface design. Originally, most early power controls consisted of switches that were toggled between two states demarcated by the words On and Off.
As technology became more ubiquitous, these English words were replaced by the universal numeral symbols 1 and (typically without serifs) to bypass language barriers. This “1” and “0” standard is still used on toggle power switches. So from the Binary Number System, 1 stands for Power ON and 0 indicates Power OFF.