Because thats the only way to convince the art world that what you're producing is art. It is for me, but then I was brainwashed at Uni. And what you said there about 'the method' - that's the whole point of performance art. A program without output would be useless. Um, rethink that.
Many programs dont have any output, but merely function.
It's what you implied by saying that its 'also about the output'. They're not bound together, you could have one without the other, I'm sure. A running software program or other computing operation. You're certainly right to think that process without output is quite restrictive. With conceptual art, the focus is on the concept, the output is merely a 'byproduct'.