Charlie was the one character with the most "near" death experiences. Or death experiences, depending on how you fall in the Big Premise debate.
Let's review the near death/death aspects of Charlie:
1. He was a heroin addict and took near lethal doses when his life turned sour.
2. He survived the plane crash.
3. He nearly got sucked into the airplane turbine (like another passenger would do).
4. He was nearly beheaded by a piece of falling plan debris (after engine explosion).
5. He survived two perilous ordeals with the smoke monster.
6. He was attacked by a vicious swarm of bees.
7. He was nearly trapped in a cave collapse.
8. He was hanged by Ethan (and had to be revived from death by Jack).
9. He nearly fell into gorge with old suspension bridge failed.
10. He was speared in the throat by a Rousseau trap.
11. He had a large bag of rocks fall on his head.
12. He survived the dynamite blast at the hatch door.
13. He survived the Hatch explosion-implosion caused by Desmond's fail safe key.
14. He survived Hurley's van plunging down a hillside.
15. He almost drowned saving a drowning woman in the ocean.
16. He was almost hit by lightning bolt on a clear day.
17. Charlie drown in the Looking Glass radio room.
Charlie was the luckiest or unluckiest man alive, again depending on one views the LOST world.
It is hard to pin down what purpose the Charlie character was supposed to bring to the table. If one thinks Desmond was the catalyst to fuel the conclusion in the sideways world, that would underestimate the role of Charlie, who got Desmond to awaken by nearly killing him by steering the rental car into the harbor (to recreate his own Looking Glass death scene).
Charlie's past was one of a religious upbringing in Northern England. It appeared that his closest friend was his brother, Liam. They created a one-hit wonder band, but drugs tore it a part. When Liam left to start his own family, Charlie was alone and consumed by his own drug addiction to the point of hospitalization. He went to Australia to reunite with his brother and rekindle the band, but he was rebuffed.
It is clear that Charlie was a follower for his entire life. He had a longing to belong to something bigger than himself. He wanted to be useful, to be needed, and to have someone who could rely upon him. That is why he gravitated toward helping Claire. From the beginning, he was protective of Claire. Then, most fans believe he fell in love with Claire because she gave him something he desperately wanted: a family. He says that the best moment of his life
was when he met her.
Like most performers, he lacked social skills. He was cheerful and liked to make jokes to ease the atmosphere. He became friends with Hurley.
If Charlie was supposed to represent a religious contrast, a crisis of self, then the series missed this plot line. He was a wayward child who did not practice his religion. His lifestyle rebelled against it. He lost his faith due to his drug obsession and band crisis, but went whether it was real or not he may have regained it as his last action before death was making the sign of the cross.
Was Charlie's death the sacrifice that saved his friends? No. Widmore's crew still came to the island. Flocke continued his rampage of terror and manipulation. Desmond's vision of Claire leaving the island with Aaron proved false.
If the island was a place of death as alluded to many times during the stories, then it is clear that Charlie died FOUR times on the island: in the plane crash, hanged by Ethan, in the Hatch explosion, and drowning in the Looking Glass. Since Charlie experienced the "most death," he was the one who was the most likely to haunt his friends like Hurley - - to be a personal messenger, guardian or spiritual protector. Since he knew about his death, that the island was death, he could accept it (which is the hallmark of awakening in the after life) which meant that Charlie was the key in the sideways story to get the people to the church (in an illogical and inconsistent way since Charlie truly awakened second to last when Claire gave re-birth to Aaron backstage at the concert).
If this was Star Trek, Charlie would have been cast as the classic red shirt. His purpose was to show the dangers to be faced by the other (more) main characters.