Because this chapter is all about 'с' and the instrumentive case. "He speaks with you" is very different from "He and you speak". The sense of the sentence would be quite the same I reckon, but technically it is different.
It is the adverb (modifying how he speaks in this case). Many adverbs in Russian are formed by just taking the masculine version of a verb and choppin' off the last letter (текнически, практически, по-детски, etc.).