why do humans use speech as a primary communication tool?
Unless citing cause, or explaining "how", questions of "why?" are imponderable. In the former case, the explanation is biologically evolutionary, i.e. some humans use speech as a primary communication tool, and, species which use speech as a primary communication tool have an increased likelihood of survival. In the case of explaining "how?" these articles might be in the neighborhood of what you are looking for, however, speech is usually covered in the philosophy of language as a more general concept of utterance which includes any speech act:
"Speech Acts and Illocutionary Logic"
"How To Do Things With Words"
"Utterer's Meaning and Intention"
"Intention and Convention in Speech Acts"
In the latter case of positing imponderables, you'll simply have to figure that out for yourself. To do so, I urge you to consider intentionality:
"Intentionality and it's Place in Nature"
of Intention and Action"
...and ponder the intentionalistic primitives of perception (sensory nervous system) and action (motor nervous system) as they relate to the physiological and psychological capacity for speaking e.g. the logical structure of speech, and, how empirical verification of such biological functions may be obtained and hypotheses regarding assessed a truth value, confirmed and advanced.