Secrecy abounds in Great Expectations. Most of the characters are harboring one secret or another, and these are revealed in due course. The theme of secrecy has two main purposes. Firstly, it heightens tension, which keeps us interested. We sense that things are not always as they seem and that there's always something going on beneath the surface. We keep on reading because we want to find out which secrets are being kept and why.
The second purpose is to give the characters greater dimension. Miss Havisham is a remarkable character as she is, but the revelation of her secret to Pip makes her, if anything, even more fascinating. And Abel Magwitch is larger-than-life right from the get-go, but again, his own extraordinary secrets add depth to an already intriguing character.