If and only if you would have used electrical heating anyway, then using a GPU's waste heat for heating your home is one of the best things you can do with it and it's "100% efficient". That is, 100% of all the energy your GPU consumes also ends up as heat in your house and it's thermodynamically equivalent to any other type of resistive electric heater.
Of course, the energy consumed by any even semi-serious mining operation far exceeds a home's heating needs.
However, resistive heating is 3-4 times less efficient then ground/air source heating, so if your actual aim is to heat the house, you can do a lot better[1]. It's just better, if you're generating heat anyway, to also use the waste heat than to dump it outside.
[1] YMMV in permafrost regions, and the hardware is too expensive for many people.
Not necessarily, a FIR heater (which is also resistive) deposits a larger fraction of its input energy in humans than a GPU cooler which heats the air which then indirectly warms the human and also everything else.
They do use little electric heaters some of the time.
And there's smaller mining operations, the economies of scale aren't that great, more the difficulty of finding GPUs. You can lend it to neighbors and give them a cut of the proceeds, you could do the whole town. So in fact ordinary mining operations waste the heat, this would not.
I want to answer the other poster who shows up dead: how can you lend a miner? Like lending a little heater. Like lending a cow, one guy needs milk the other wants the meat and hide. So if you already have all the electrical heat you really need for your house but want to mine more, you won't benefit from more heat for your own house. Whereas maybe across the street a neighbor is still using little electrical heaters a lot, doesn't understand crypto very well. So they work something out, the neighbor gets heat, pays the electric bill, and gets a cut of the mining, and the original miner provides the GPU through his contacts and gets a share too. So that way the heat doesn't go to waste, and the miner can scale.
Clarifying for alisonkisk, lend means 20 cards instead of 10 you need for heating, lending the other 10 out to a neighbor for his heating. Economies of scale, for instance by buying more cards at a time at a better unit price.
How can you lend your miner to a neighbor for heat? You all need heat at exactly the same time. Are you going to move the rig to a different home every hour?
Well right, you can't. Very good point. But perhaps you have guests for a month and when they leave they have guests, that means more of the house has to be heated instead of sealed off. I've seen that in huge houses.
Of course, the energy consumed by any even semi-serious mining operation far exceeds a home's heating needs.