The data on each pixel must be read out, and then transmitted from (I'm presuming) a purely analog chip over to a digital chip for encoding and digital read-out. Perhaps there is some digital circuitry over on that analog imager chip, but most certainly there HAVE to be amplifiers of some type to drive the data off-chip.
And when you go to power up those amplifiers...
...you're going to have a drop across some sort of resistance, and that's power, and that will get radiated off as heat. Heat for both the imaging chip, and probably heat radiated to other parts of the instrument.
So how this thing is going to keep itself so incredibly cold? I have NO IDEA.
Wow Cool. O.O *stares into space*
This may be a useful article about keeping it cool:
https://webb.nasa.gov/content/about/innovations/cryocooler.html