The first sidewall reflection point matters in the design of a recording studio control room. Those I have been involved in used angles which direct the reflections away from the normal listening position.
In rooms where that’s not an option, apparently the first sidewall reflections still matter... or at least, what one does at these locations matters. @erik_squires on the subject:
"To my experience, as others here have noted, diffusion at the 1st reflection points are a better idea [than absorption]. They enhance imaging."
One significant difference between diffusion and absorption is, diffusion doesn’t significantly change the spectral balance of the reflected energy, while absorption does. And absorption keeps on having its effect on everything that hits it, not just those first reflections, for better or for worse. Both reduce the amount of early sidewall reflection energy which arrives at the listening location, but diffusion does so more benignly.
Early sidewall reflections have benefits. They DO increase the apparent source width... in other words, they can make the soundstage wider, but some image precision and soundstage depth is lost in the process, along with some clarity. So it’s a tradeoff. If not absorbed, that first sidewall reflection energy lives on to come back as later reflections which convey spaciousness (hall ambience) with no significant downside.
Earl Geddes on the subject of early reflections, which typically include the first sidewall reflections:
"The earlier and the greater in level the first room reflections are, the worse they are. This aspect of sound perception is controversial. Some believe that all reflections are good because they increase the listeners’ feeling of space – they increase the spaciousness of the sound. While it is certainly true that all reflections add to spaciousness, the very early ones (< 10 ms.) do so at the sake of imaging and coloration... the first reflections in small rooms must be thought of as a serious problem that causes coloration and image blurring. These reflections must be considered in the [loudspeaker] design and should be also be considered in the room as well."
Duke