To answer the question of why not leave it in one gear, along with all the other answers like weight (who cares), simplicity, easy to maintain, etc. is the fact that derailleurs means more **** the chain has to go through, slowing it down along the way. If you remove the derailleurs the chain only has to go around a cog and a chainring, freeing it up of the resistance caused be derailleurs, thus making it easier to pedal in the same gear.

On top of all of that, it looks really pretty.