It kills people, it read people's minds, what is it's actual job though?

Danille (the french chick) called it a security system

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Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.

According to the Map on the back of the blast door and a number of other references ( indirect and direct) there is an area called dark territory in which the Smoke/cerebus/security sustem has been known to roam. why the island need a security system and whether or not its operating on its own or by some mastermind .... we dont know definitively... and i dont read spoilers.

i have attached a few links to help you.

Cerberus - Lostpedia
Dark Territory - Lostpedia

It's just there to scare the Bejeebers out of them!!

"Protect the Island."

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" I push this button every 108 minutes, I don't get out much."

"Theres a time for living low down, theres a time for getting high. A time for peace, a time for fire. A time to live, a time to die."

The real art of conversation is not only to say the right thing at the right time, but also to leave unsaid the wrong thing at the tempting moment.

to scare me even though it's just a tv show

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I believe, it is dark matter, leaking through the rip in the space time continuim, just to mess with us. i also believe I'm having a nervous breakdown, my 19th.

Protect the island from what though Tess, us? I really don't know how the producers are going to explain 'ol smokey, it will be interesting to see, esp from our postion on a Hawaiin beach watching the finale on a huge movie screen!!! oh yeahhh