Hi
The Tenor voice can be shown in several ways. Older music sometimes has it written on the "Tenor Clef" (one of the positions for a moveable C clef), if accompanying a Bass vocal part, it might be notated on a Bass Clef. When notated on it's own it is now often notated on a Treble Clef, notated an octave above the actual sung part. This can be indicated in several ways, the most common of which is the addition of an "8" at the bottom of the Treble Clef symbol. A less common way is to use a double Treble Clef.
The reason is simple. Music is easier to read within the lines of the stave. If you were reading music that was for the large part based on ledger lines it could be quite awkward to read. Using the solutions I just mentioned, that is avoided.
Cheers
John
