Why there are 2 sets of blades in helicopter?

I will assume by "sets" you mean "pairs". The truth is, some helicopters only have one set. Some have two. Put very simply, the more blades you have, the more lift you can generate, and so the more hauling capacity you have. Eg the Chinook has TWO rotors, each with four blades. It's used to lift some very heavy objects (humvees, many personnel etc). Whereas a small R22 only has two blades, since it's not designed to be a cargo lifter. The Main rotor is generally the one that is the most forward and creates most of the lift and general directional control (forward, back, left and right banks) Where as the tail rotor provides for a counter force to the torque created by the main rotor and also give you the ability to do what is called a balanced turn (means you won't feel like you're sliding through the air)