Computers are dumb and need to be told how to take the data of an image (stored as a long series of 1s and 0s in memory) and draw it on the screen so you can see it. The people writing the software to do that needed an image to test with, just to make sure everything was working right.
Either because they were a bunch of lonely geeks in the 70s or they didn’t have any other good photos to scan in, they used a headshot of a PlayBoy model. They couldn’t have known that it would effectively become one of the first digital memes, meaning it’s still semi-frequently used by graphics programmers (professionals and enthusiasts).
I can’t claim to speak on the model’s motives, but it’s not hard to imagine that having their headshot used in perpetuity without consent would make someone uncomfortable.