As the previous answer states, there are loose autobiographical elements to Great Expectations. David Copperfield, another one of Dickens' "orphan" novels is closer in reality to Dickens' actual upbringing.
In regards to other reasons for Dickens' writing Great Expectations specifically, he did desire that the novel be a social commentary on life in Victorian England. He uses Pip's character to identify the hypocrisy of this supposed "Christian" time period in England and also to demonstrate that those who were supposed to be ladies and gentleman often behave with the least class, while the lower class (Joe and Biddy represent this theme) illustrate dignity and--in Biddy's case--a discerning view of human character.
Dickens' also uses the novel to comment on debtors' prison and inequitable treatment in England's justice system (Compeyson and Magwitch).
Charles Dickens is both famous and notorious for using his writings as a way to cathartically express the rough upbringing he had, and the pressures he had to endure earlier in his life when his parents (whom indeed had great expectations for him) all of a sudden failed him, and ended up basically ruining his childhood by making all sorts of bad choices for the family unit.
Dickens did write the novel as an outlet. He was already growing tired of his married life, of his family, and still he was holding grudges from having his childhood spent in a forge (or work house) when he was a teenager and wanted to be something special. All because of his parents' inability to maintain a budget and lose all they have. Hence, throughout his entire life, Dickens has tried to vent out his early frustrations which, unfortunately, even at the height of his career he was still not able to shake it off.