It is all about system matching. BUT IMO, preamps do more that what has been already stated above. A good preamp will add dynamics, loud to soft to loud contract, speed, soundstage dimension - front to back - side to side, tone, separation of vocals and instruments, flow of music, definition, details, clarity, float high frequencies, provide a more beautiful mid-range especially with female vocals, woodwinds, and strings, a more realistic sound, etc.
Happy Listening.