That is, if the stem of the verb doesn't already end in an s-sound. "z" is spoken like "ts" in German, so there is indeed an s-sound in the end of the stem "benutz". Hence you only add a "t" for the second person singular. Some other examples are: "du isst" (from "essen" = "to eat"), "du weißt" (from "wissen" = "to know"), "du sitzt" (from "sitzen" = "to sit"), "du reist" (from "reisen" = "to travel").

I hope I could be of help.