Before I answer the question why you still need acoustic treatment?
I think its important to explain what treatment is and the problems that one can run into if not aware of the issues.
Acoustically treating a room refers to the process of optimizing the acoustic properties of the room, especially in recording and other sound studios. The goal is to make the room sound more neutral. When sound waves reflect off surfaces and back into each other, they can cause all kinds of audio issues.
Such as increase in modes or a cancellation in frequencies this is caused by the room dimensions and the distance from the sound source. for example if you have you singer in the middle of the room. you are less likely to get what is referred to as combing or comb filtering. which can cause your recording to sound weak or muddled(comb filtering occurs when two or more waves are mixed together at a slight delay. The result in frequency graph looks like a comb meaning some frequency are in phase, some are cancelled out and are partially out of phase with each other)
but if the room is untreated you can still get a boost is frequencies or worse a cut or cancelation of the frequency. This is caused by the sound hitting the surface and bouncing back on and adding or cancelling itself out as sound is omni directional (i.e it travels in all directions). It will react with all the surfaces in the room and the bounce back and forth an if it dies out what we commonly refer to as reverb.
In a untreated room this can be harmful to the recording as the mics are un-objective and especially in the case of a large diaphragm microphone the sound the room sound will be captured witch might damage the recording.
In the cause of mixing it is equally important to understand what is happening in your room as small changes make a big difference to the end product. if one where to boost in a room that is cutting the frequency you where perceiving the monitors to be telling you might think twice once you listen to it in you car. leading to more trips back and forth checking your mix cause you just not sure. amongst other acoustic issues that can arise in a room.
So how can we solve this?
There is a ton of resource on the internet on how to treat a room to make sure your room is ready to work in with a bit more confidence and not have to keep running back to the studio to get your songs recorded cause of the room sounding bad.
Here are some tips I have used in the past to help recording and mixes go by more smoothly
Understanding that there are two main ways to treat acoustics namely absorption and diffusions once you understand these two principles you can help you recording and mixes out a lot.
1 use a combination of absorption and absorption in you room to give you a more even modal displacement
2 treat ceiling and floors as they are surfaces we tent to for get about and in most cases some of the first reflection point
3 listen to the room and find where I sounds best for your recording
4 be mind full of you sweet spot when you mix it is very easy to losses it when you just changed something on you analogue gear and try come back to it having a remember you should have set up you monitor in a equilateral triangle once you have found your first reflection points using the mirror trick
5 treat your first reflection points
6 try avoid square rooms
7 use treatment to add in the recording to much absorption and make the room sound dead, to much diffusion can cause more harm then good making things sound bigger than the really are or odd
8 place bass traps where you can corners add to bass build up and are more prone to comb filtering then the rest of the room 9 be creative in how you use the room to your advantage (i.e use sound blankets and some mic stands to change the dimensions of the room)