Well, one reason to believe otherwise is the simple fact that we are talking about consciousness.
If consciousness were just a by-product (like steam in your example) then there would be no physical drive to talk about it.
If consciousness were just a by-product (like steam in your example) then there would be no physical drive to talk about it.
I firmly believe that most if not all interpretations of the 'world' is processed and complete before it even reaches perception. How I recognize objects, how I feel at the moment, and my literal thoughts themselves. But I cannot deny that the brain activity that processes stimulus into interpretations into perceived experience is also very aware of the perceived experience.
Perhaps it's as though our brain has a functional programming style in that various stimulus is operated upon somewhat independently by different regions of the brain, but the operation of synthesis takes the previous perceived experience (that is, the output of the previous synthesis op) as part of the input along with the various outputs of newly processed stimulus to create a new synthesized state that has reliable continuity.
It could be that consciousness is simply an emergent property (a by-product) of the communication of billions of neurons that are connected in trillions of ways. Where neurons are emergent of chemical phenomena which are emergent of atomic phenomena and so on.
I'm probably greatly simplifying the dependencies of phenomena, but I think that consciousness is most definitely based upon physical processes. The current evidence makes me believe that consciousness is a by-product of evolution, which is a by-product of the conditions on our planet.
So it goes.
Touch hot flame -> Feel sensation of pain -> Move hand
It's...
Touch hot flame -> brain processes event and sends signals to move hand -> You experience all of this but it had no relevancy to the chain of events.
EDIT: This does beg the question 'why do we have consciousness, then?' or slightly related 'how could we talk about it?'. Which are interesting questions, and probably explained by darwinian processes, but don't really get at the heart of it. My thoughts anyhow.
All of this is at least intuitively easy to explain with added language and complexity in the brain. So like the chain would become:
Touch hot flame -> brain processes event (and then does complicated language processing, and complicated memory and emotion processing) and THEN sends signals to move hand or not move hand -> You experience all of this but it had no relevancy to the chain of events.
Where I would ponder the relevancy of consciousness is when it comes to attention and remembering things. Just from personal experience, I can read a book, watch a movie or listen to music and I will not remember a thing if I wasn't aware of it. Is awareness and consciousness the same thing? I don't know, but at least we are conscious of everything we are aware of, by definition, and not necessarily vice versa.
Also, how would the ability to think evolve in our species, if our thoughts never had an impact on our behavior? Then they could not provide any evolutionary advantage.
It is possible (and quite likely) that some weaker version of that statement is true; e.g. that there are many things that brain does independently on our thoughts. But claiming that there is "no" impact is just trying to be edgy. Yes, such absolute statement sounds more interesting, but it is wrong.
Some questions can't be answered through language they can only be conceptualized through language and that's a very important distinction IMO.