Playing 2 hands - When and Why?

I would like some advice on playing 2 hands. When should it be done? Is it just a way to get more money out on the table? Or is there othere reasons? Does it matter how many players are at the table? (meaning - is there a time when you play 2 spots if your the only player?). I use a simple level 1 unbalanced and I remember hearing somewhere that some will play 2 hands not only in positive counts, but in negitive counts as well to eat up cards. Is this correct? LAst but not least, is there a time when a BS player should play 2 hands?

Thanks for your time and advice.

AK