The decibel unit was invented by Bell Telephone Labs, for telephone amplifiers which were driving speakers, to analyze how much power is available, and how much power was lost in getting the signal to the speaker, and producing further a pressure change.
In audio, 1dB is the smallest volume change which can be heard in a quiet environment. 1dB represents an approximate change in pressure of 12%.
In audio systems was used 6dB, which represents twice (half) of pressure increase.
By analogy with this "half/twice" from audio, engineers agreed to use 3dB (half power) in electric circuits.