The ships computer, Eddie, becomes unable to accept new processing requests because it is running at full capacity after being told to make "Tea, with Data's breast milk" by William Riker.
I know it's a meme but there's an actual answer: because humans have them, and Dr. Soong wanted Data to feel human. That was the whole point of the project from what I understand.
Red Alert!!!! ... Computer is diverting power from life support! .... the warp core is failing and being drained of power ... lights and control panels all across the ship start flickering and become non functional
There's no photo evidence of it I can find, and I've been given a good reason to search for "start trek data nipples". You all saw it. This was justified.
But seeing as some people are super insistent, I'll have to conclude that it's A CONSPIRACY!!!
Birds aren't real, the earth is hollow and Data has no nipples!