I am sure they explain on the maker’s web site, however I think it is to do with adding carbon, so you get a balanced mix. I think they say you can use shredded wood. I considered one, but they are very expensive, and I worry about their longevity given the material they are made from. As said, size does matter, a big pile can get very hot. I created a huge pile when clearing my garden, and the inside turned grey and steamed, it looked like burning ash and was too hot to touch. I bought ~1000L wooden compost bins, which seem to do the trick. The advantage of the hot bins is that because they are insulated the outside of the compost will also get hot.
The only reason I might consider one is that the temperature will kill weed seeds, so I could use pony poop from the local roads. The last time I used pony poop, I ended up with huge amounts of nettle seedlings in my veg beds.