To buy more processing power
Or to have more power over the production of processing power
Processing power being the original use for enslaved humans instead of batteries/ power source - it being changed because it was thought processors were too complex for the audience to understand.
This is the best tl;dr I could make, original reduced by 87%. (I'm a bot)
What about mining Bitcoin in an ideal scenario, where the generators were perfectly efficient and able to harvest all 80 watts of excess heat produced by the body? Maybe something like the Matrix, where humans are immersed in a big vat of goo that harvests all their energy?
If you wanted to speed things up and mine 1 bitcoin per month, you're going to need about 320 people lying still around the clock with a perfectly efficient system.
To do 1 bitcoin per month with IoHO's tech-which in terms of efficiency is on par with most affordable wearable thermoelectric generators available today-you'd need around 44,000 people providing their body energy 24/7. Of course, the IoHO isn't advocating for a future that looks like the Matrix.
Worth noting that the Matrix was originally supposed to be about harvesting CPU power from humans, not about using us as energy batteries. Concept was changed by the studio to make it easier to understand as this was all before cloud computing.
I think that was dumb, but must remember audiences were so different in 1999. The Matrix was incredibly complicated for most people then, and they didn’t have the benefit of all the summaries and even academic texts we have on it today.
Also If you go back and read reactions to the marketing of the Blair Witch Project at almost the same time, many people, while not believing it fully, had a tough time understanding it was 100% a marketing campaign.
> An adult human body generates approximately 100 watts of power while at rest, and about 80 percent of this power is wasted as excess body heat. [...] on average the volunteers each contributed about 0.6 watts/hour of energy.  
> What about mining Bitcoin in an ideal scenario, where the generators were perfectly efficient and able to harvest all 80 watts of excess heat produced by the body?
For this calculations, it's not enough to use the energy/power numbers. You must consider the entropy too.
The maximal efficiency is 1 - T_cold / T_hot (in Kelvin). In this case, T_cold is the temperature of the environment, that we can assume is some comfortable value like 20° (293K, 68°F). And T_hot is the body temperature that is approximately 37°C (310K, 96.8°F). So the maximal efficiency is 1 - 293K/310K = 5.5%.
If the heat produced by the body is rest is 80W, the maximal amount of it that can be collected and transformed to electricity (in a very optimistic scenario) is 4.3W. They are collecting only 0.6W. It's imposible for theoretical reasons to collect the 80W. 
 I'm not sure how the other 20W are dissipated. As heat?
 "0.6 watts/hour of energy" doesn't typecheck
 You can give the volunteers some drug to increase the temperature to 40°C (313K, 104°F) while keeping them in a freezing environment 0°C (273K, 32°F) This will increase the maximal theoretical efficiency to 12.7% (that is still much less than 100%). Anyway, no sane ethical committee and/or insurance company would approve the project.