Most rooms don’t need acoustical treatment.

The "knowing how a car works" is still a valid analogy.  While many have a general understanding of how engines, brakes and steering work, very few of us are mechanical engineers and have a detailed understanding of the design details and why certain choices were made. And keep in mind that even engineers tend to work in very specialized areas -- you don't hand the development of the car's suspension to the engine team.   

The same is true with many of the technical devices we use daily. One does not need to be an electric engineer or app programmer to use our cell phones or laptops.  

The same is true of acoustics and room treatment. It is a discipline that is well understood by the specialists in that area. However, what is obvious to them may be much less so to the rest of us.  But that is the beauty of audio as a hobby -- it is easy to experiment.  If you get good results, great! If you make things worse, they can generally be easily undone.  And no one is is danger or has their life ruined by the process. 

@jumia -- I think you misunderstand the general direction of this thread.   

The subject of room acoustics is well understood by those who have studied it extensively. However, it is not an issue where an amateur can do a five minute Google search and have any assurance that they've come up with the correct answer for their particular situation.   

There are lots of things like this in life.  You want your airline pilot, doctor, structural engineer, and so on, to have specialized knowledge and training for their job.  It is certainly easy enough to look up info about all of those subjects on the internet, but that hardly means that the person who does that is ready to fly a plane, operate on a person, calculate load bearing specifications, or do the many other things that people spend years training for.  

Acoustics, and all those other things, still may be mysterious to you, but that doesn't mean that others out there don't have a good handle on things.  

But, that's the nice thing about this hobby. You can experiment to your heart's content with no concerns about the plane crashing, the patient dying or the building collapsing. Who know, you may even end up improving the way your stereo sounds for you!