Within the Compson family, Benjy is the one to foresee the most likely things to happen in the future, and he actually silently predicts the failure of the family through his observations of their interactions. He senses a lot of what will happen in the future, which none of the other characters are capable of because they are so caught up in living their own lives. Benny, however, can't understand any of these things he's discovering, and therefore can't relay his discoveries to those that matter to him. All he understands is chaos, which he expresses in his moaning. The others, who are so obsessed  with their own lives, blame Benjy for the imminent downfall, even if they may not see it coming yet. This shows that the Compsons are uniquely selfish and unable to truly understand Benjy and his very obvious and apparent issues. They are too caught up in their own lives to understand they are harming themselves in everything they do.
Benny is dependent on consistency and family because he can't rely on himself, with all of his issues and the blame of others for the failure of their family. Because everyone else has to care for him, when there is chaos or worry going on around the family, it affects him much more than the others because he can't process or understand what's going on in the same way or capacity as those around him. He looks back on happier memories, when the children were young and carefree, to avoid the stress of their adult lives, much of which is forced upon him. This shows that the Compson family, and maybe all families in general, grow more tense and further apart as they grow, because issues, misunderstandings, and arguments begin to accrue. This is specifically difficult for Benjy because as a child, it is safe to say maybe his disabilities were more acceptable, and he thought of those times as more laid back, as times where he felt more loved and understood. Now, however, as an adult, Benjy is expected to do much more than he is capable of, and he can't understand nor keep up with it, and this could make him retreat back to the past. The more stress in the family, the more Benjy retreats. This gives a very unique dynamic to the South, especially to families of the South, as if expectations grow exponentially with age. It even suggests that maybe Faulkner was representing the South as ignorant and selfish, as the Compson family is.
Benjy is most likely meaningless to the Compson family, but to the readers he is greatly symbolic. He represents the struggles and burdens of the Compsons, by singlehandedly encompassing them all. The family uses him as an excuse for all of their mistakes, and he takes it on because he has no way to resist or fight back. He is the mess that the Compson family has always been, and will continue to be. His condition is unchanging, as is their imminent downfall. He is a symbol for what is to happen within the family, and how easily it can fall apart, with almost no one paying attention.