why DID we evolve into humans?

Hello tomwillrep,I think the problem you're confronting is in describing evolution as being "motivated" toward some goal. Simplistically stated, we evolved into what we are by a process of natural selection. There wasn't a stated goal to where we would be headed. It just happened that those traits that we carried either helped us adapt to our environment or were selected out. Our path is just one of a multitude that happened to survive. There was a time when there were different forms of bipedal hominids, robust plant eating forms with thick jaws and molars for crushing plants and nuts, smaller types which were more generalized, etc. Our path was one toward a more generalized, bipedal, large brained form.
The apes that are alive now specialized along a different path than ours, they remained arboreal. (as far as I know they didn't evolve from non-arboreal bipeds). The point i guess i'm trying to make, poorly that is, is that there is no "directive force" or motivating force" for evolution. It just uses what we have or discards it, over periods of time depending on what "works". As far as voice boxes are concerned, apes don't need them to survive. They are successful (so far) without them. somewhere along our lne of descent we evolved voice boxes which we found a need for. Some form of communication, I'd almost argue, is a necessary componant of survival. We happened to use our voice box as one way of communication.
Who knows if our experiment of form will survive? It may come to be that large brained culturally advanced bipeds will be selected out, through their own devices (war, overgrazing ) or by some other agent (disease, catastrophic event/s, extreme environmental change).
I'm not sure if I answered your question, but it did seem there were some mistaken ideas about evolution in your question.