That depends. If they happen to heat a small area around the user, then it will allow the ambient room to be at a lower temperature overall, leading to a net power savings because (counterintuitively) of lower efficiency.
There are a number of factors influencing that conclusion. Many people opined on this issue a little while ago, "heating people rather than spaces" https://news.ycombinator.com/item?id=9034678
Could be. I'm just observing that there is a law of conservation of energy and that heat can be useful, so the question of using old computers in terms of environmental impact is not as clear cut as many internet comments suggest.
