what is this for?

It's called Shimano airline. and it's a Hydro shifter and mech.

i heard that it dosent relly works well, and it just adds weight

pneumatic, and it actually works much better than a derailler, expensive, but still heavier. you can cut on wieght by braizing your tube and inflating the tube to save the weight of the tank.

.....this si just a bike..NOT a truck...