In every definition of a blackbody I read, it always mentions how a black body is something that emits and absorbs at every wavelength. I understand why it is important that it emits at every wavelength, since otherwise we wouldn't get the blackbody spectrum.

However, why is it important that it absorbs every wavelength? Will this affect the output? Can't I supply it energy by some other means (like giving it a power supply?). In the common hole in the box, we are not providing every wavelength but rather just giving heat directly and getting a black body output.

It seems that the absorption part of the definition has little use - why do we keep saying it then?