what is  the objective of this attack and why do people use it

Do you mean the Sicilian Dragon? This? If so, the idea is a razor sharp variation where one step is a step that loses you the game. If Fischer's quotation "Chess is life," is true, then you may well be sleeping in an aquarium when the last bit of wood is pushed.

Motifs include an exchange sacrifice on the c-file (Your Rook for his or her c3 Knight, I believe), dark squared play (where neither player will give up his or her DSB unless it is an exchange, most often done when White has a Queen on d2 and Bishop on e3 and plays Bh6 to exchange for Black's fianchettoed Bishop), mutual pawnstorms, and players with weak bladders having to read a disclaimer before touching a piece.

Objective: Checkmate white

Used Because: It sometimes works

The Dragon got its name because some early master thought its rippled pawn structure - a6,b7, d6, e7, f7, g6, h7 - resembled an undulating Chinese Dragon.  The sharp play it involves may have contributed to that impression.

Much of chess involves balance or the lack thereof.  It is in the imbalances of position where winning chances are found.  Where there are no imbalances in a position, a game between players of reasonably close strength will be a draw.  The more imbalances, the more likely the game will have a decisive result. 

The Dragon may be the most imbalanced opening in chess (at least where Black has good chances, too).  Decades of grandmaster experience have developed many razor-sharp lines where one small error by either side can cost the game.  So it is exciting for the players, and often also for the spectators!