Why does Magwitch help Pip?

Magwitch helps Pip out of gratitude for the food and fear-sharing on the marshes — and out of a hunger to own a gentleman as proof he was not only a convict. The help is real care mixed with fantasia. He works in exile to invent Pip as living revenge against a society that marked him as trash while letting Compeyson’s manners pass.

That double motive is why the revelation is devastating rather than simply touching. Pip must accept money that bought his snobbery, then learn to care for the man without accepting the fantasy. Guilt-and-gratitude is the concept; this question is the entry point for readers who only remember “the convict paid for everything.”