If you have a nice system why do you really need room treatments?

You missed the very important time element….. the ear - brain needs X ms to ID….

RT where T = Time

Many of the Products produced to be used in a Audio System are tried during R&D stages in dedicated demonstration spaces.

These Spaces are all about not producing sound that is being corrupted by the types of condition that are to be met in a Typical Listening Environment used by a purchaser of goods.

Once a Product is purchased, there are some products that are not designed to be at their best presentation when used in their new environment. As the end sound can't deliver as it had at the stages during the design, due to the influences caused by the Room and Set Up within the Room.  

Room Treatment is a simple method to attempt to enable a end sound be produced that is a resemblance of a Sound be created during the design stage.

Note: resemblance to the produced sound is the key word not, a match for the sound produced during R&D Stages. 

To not attempt to optimise the room for the audio equipment in use is absolutely fine, music is still being produced.

To attempt to optimise the room for the audio equipment being used is also absolutely fine, music is still being produced, but possibly ? with less influences produced by the room and room set up.

Reflections , I have 10 , 1st reflections from the front, in corners Bass traps which can create boominess ,then directly behind ,carpeting Always recommended in front at least for slap echo off floor onto ceilings . Some people with $$ like my brother have the whole room treated and looks neat it’s a dedicated multimedia room but that’s over $10 k . 2x4 room panels are under $100 each 2x2 behind on walls is good also. They come in different  color fabrics , even with art work on them ,which I plan on buying to make the man cave more presentable to the Mrs..

the room is more calm and refined , plants too help as defusers ,you see them at shows.

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Here's a very basic analogy: you'll easily see the difference between a mediocre tv and a state-of-the-art tv. But if the light in the room is subpar and the sun is shining directly on your fancy tv, its image won't be as good as it is capable of being. Room treatment helps your system to live up to its full potential.

If you have a nice system why do you really need room treatments?

to make it sound even better, to make it eveneasier to get great sound.