A friend of just bought a 5 string bass and I was wondering why a five strings? What is the fifth one for? What music style and so and so...

Thanks in advance.

The 5th one is a low B underneath the E. It expands your range, and if that's his thing, play heavier riffs (although you can play heavy riffs on an E, I know...).
Yeah, it's just for those extra low notes, I've only seen them used in metal, and Primus? (6-string in that case?) But there is no reason they can't be used in other genres...
And what styles?
From jazz, listen to alain caron to metal to everything, it just depends on the octave you want the sound to come from
It is either a low B or a high C string (if it is above the G).

They give the bass a larger range, and most vitally, they give economy of motion which allow you play a wider range of notes without moving your hand. For example, now you can play 2 octave scales with only one shift of your hand, while having 4 strings requires quite a few more shifts.
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