The 5th string on a bass gives you the option of either a Low B or a High C. If you play original music it's just like having another color on your palette. That's why there are also 6 and 7 string basses.
In a lot of the new music genres of today's youth such as Sludge and Doom Metal, they are tuning their basses lower and lower. (These are the things you learn hanging out at
http://www.talkbass.com/ )
They are now tuning basses down to low C below the E! While I don't partake in or enjoy this type of music, I do like to be informed as to what's going on in the world of music as the envelope is continually pushed. In case anybody is interested, here are some examples:
https://www.youtube.com/watch?v=poUWux2TjxE
You know how it is, my parents didn't like the music I listened to as a kid and on it goes for each subsequent generation.
Now since I play in a cover band and most of the music we cover is form the 50' to the 90's most, if not all of it was recorded with a 4 string bass so I have no need for a five string. But a lot of popular music is in 'drop-d" tuning for the ease of playing power chords and a 5 string with a low B makes a lot of sense there.