It's also used for the Tenor voice... again, it's often omitted.

The reason for its use is simple - if written in the bass clef, most of the notes would be up in the 'leger lines' (French origin, roughly meaning 'flying lines').

There is another clef, drawn as an odd looking K, in which the line passing through the centre of the K is Middle C - often referred to as the 'Viola clef', the viola being the instrument which is almost always written in that clef.

The complication is that the K clef can move - middle C could be the middle line, or any other line for that matter!

Just as a matter of interest, the treble clef started as an uppercase G, and the bass clef started as an uppercase F. The curl of the treble clef goes around G, and the spots (originally the horizontal lines of the F) are on either side of the F line.