Because it allows for some rather hair-raising combinations like this one.
Daniel Stellwagen v Viswanathan Anand, 2009
What the heck just went on in this game?
Absolute chaos on the board occurred after Anand sac'ed his Queen for 3 rather uncoordinated pieces with his King left uncastled. This game is truly beyond me.
The problem with Sicilians is that quite likely you need to be good at tactics, and some of the gameplay can be on the knife's edge and thus, it is very easy to lose your way.
Take a look at this game. I am White.
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Black just played a seemingly innocuous move 1.... Nd7.
This looks like a typical Sicilian formation, right? Yes but....
Incredible as it may seem but Black is totally lost in this position with just that one move. Can you figure out why and what is the critical move White should play after that?
Highlight between the brackets for the answer.
[Black now loses instantly after:
2. exd6 Bxd6 or Qxd6 (doesn't matter)
3. Nxe6!! and Black must lose the Queen+more material to stave off mate]